diff --git a/R-package/src/Makevars.in b/R-package/src/Makevars.in
index 808960319..37511ec62 100644
--- a/R-package/src/Makevars.in
+++ b/R-package/src/Makevars.in
@@ -99,6 +99,7 @@ OBJECTS= \
     $(PKGROOT)/src/logging.o \
     $(PKGROOT)/src/global_config.o \
     $(PKGROOT)/src/collective/allgather.o \
+    $(PKGROOT)/src/collective/allreduce.o \
     $(PKGROOT)/src/collective/broadcast.o \
     $(PKGROOT)/src/collective/comm.o \
     $(PKGROOT)/src/collective/tracker.o \
diff --git a/R-package/src/Makevars.win b/R-package/src/Makevars.win
index 43bfcf7c1..611cff874 100644
--- a/R-package/src/Makevars.win
+++ b/R-package/src/Makevars.win
@@ -99,6 +99,7 @@ OBJECTS= \
     $(PKGROOT)/src/logging.o \
     $(PKGROOT)/src/global_config.o \
     $(PKGROOT)/src/collective/allgather.o \
+    $(PKGROOT)/src/collective/allreduce.o \
     $(PKGROOT)/src/collective/broadcast.o \
     $(PKGROOT)/src/collective/comm.o \
     $(PKGROOT)/src/collective/tracker.o \
diff --git a/src/collective/allreduce.cc b/src/collective/allreduce.cc
new file mode 100644
index 000000000..6948f6758
--- /dev/null
+++ b/src/collective/allreduce.cc
@@ -0,0 +1,90 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#include "allreduce.h"
+
+#include <algorithm>  // for min
+#include <cstddef>    // for size_t
+#include <cstdint>    // for int32_t, int8_t
+#include <vector>     // for vector
+
+#include "../data/array_interface.h"    // for Type, DispatchDType
+#include "allgather.h"                  // for RingAllgather
+#include "comm.h"                       // for Comm
+#include "xgboost/collective/result.h"  // for Result
+#include "xgboost/span.h"               // for Span
+
+namespace xgboost::collective::cpu_impl {
+template <typename T>
+Result RingScatterReduceTyped(Comm const& comm, common::Span<std::int8_t> data,
+                              std::size_t n_bytes_in_seg, Func const& op) {
+  auto rank = comm.Rank();
+  auto world = comm.World();
+
+  auto dst_rank = BootstrapNext(rank, world);
+  auto src_rank = BootstrapPrev(rank, world);
+  auto next_ch = comm.Chan(dst_rank);
+  auto prev_ch = comm.Chan(src_rank);
+
+  std::vector<std::int8_t> buffer(n_bytes_in_seg, 0);
+  auto s_buf = common::Span{buffer.data(), buffer.size()};
+
+  for (std::int32_t r = 0; r < world - 1; ++r) {
+    // send to ring next
+    auto send_off = ((rank + world - r) % world) * n_bytes_in_seg;
+    send_off = std::min(send_off, data.size_bytes());
+    auto seg_nbytes = std::min(data.size_bytes() - send_off, n_bytes_in_seg);
+    auto send_seg = data.subspan(send_off, seg_nbytes);
+
+    next_ch->SendAll(send_seg);
+
+    // receive from ring prev
+    auto recv_off = ((rank + world - r - 1) % world) * n_bytes_in_seg;
+    recv_off = std::min(recv_off, data.size_bytes());
+    seg_nbytes = std::min(data.size_bytes() - recv_off, n_bytes_in_seg);
+    CHECK_EQ(seg_nbytes % sizeof(T), 0);
+    auto recv_seg = data.subspan(recv_off, seg_nbytes);
+    auto seg = s_buf.subspan(0, recv_seg.size());
+
+    prev_ch->RecvAll(seg);
+    auto rc = prev_ch->Block();
+    if (!rc.OK()) {
+      return rc;
+    }
+
+    // accumulate to recv_seg
+    CHECK_EQ(seg.size(), recv_seg.size());
+    op(seg, recv_seg);
+  }
+
+  return Success();
+}
+
+Result RingAllreduce(Comm const& comm, common::Span<std::int8_t> data, Func const& op,
+                     ArrayInterfaceHandler::Type type) {
+  return DispatchDType(type, [&](auto t) {
+    using T = decltype(t);
+    // Divide the data into segments according to the number of workers.
+    auto n_bytes_elem = sizeof(T);
+    CHECK_EQ(data.size_bytes() % n_bytes_elem, 0);
+    auto n = data.size_bytes() / n_bytes_elem;
+    auto world = comm.World();
+    auto n_bytes_in_seg = common::DivRoundUp(n, world) * sizeof(T);
+    auto rc = RingScatterReduceTyped<T>(comm, data, n_bytes_in_seg, op);
+    if (!rc.OK()) {
+      return rc;
+    }
+
+    auto prev = BootstrapPrev(comm.Rank(), comm.World());
+    auto next = BootstrapNext(comm.Rank(), comm.World());
+    auto prev_ch = comm.Chan(prev);
+    auto next_ch = comm.Chan(next);
+
+    rc = RingAllgather(comm, data, n_bytes_in_seg, 1, prev_ch, next_ch);
+    if (!rc.OK()) {
+      return rc;
+    }
+    return comm.Block();
+  });
+}
+}  // namespace xgboost::collective::cpu_impl
diff --git a/src/collective/allreduce.h b/src/collective/allreduce.h
new file mode 100644
index 000000000..e3f8ab5b8
--- /dev/null
+++ b/src/collective/allreduce.h
@@ -0,0 +1,39 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#pragma once
+#include <cstdint>      // for int8_t
+#include <functional>   // for function
+#include <type_traits>  // for is_invocable_v
+
+#include "../data/array_interface.h"    // for ArrayInterfaceHandler
+#include "comm.h"                       // for Comm, RestoreType
+#include "xgboost/collective/result.h"  // for Result
+#include "xgboost/span.h"               // for Span
+
+namespace xgboost::collective {
+namespace cpu_impl {
+using Func =
+    std::function<void(common::Span<std::int8_t const> lhs, common::Span<std::int8_t> out)>;
+
+Result RingAllreduce(Comm const& comm, common::Span<std::int8_t> data, Func const& op,
+                     ArrayInterfaceHandler::Type type);
+}  // namespace cpu_impl
+
+template <typename T, typename Fn>
+std::enable_if_t<std::is_invocable_v<Fn, common::Span<T const>, common::Span<T>>, Result> Allreduce(
+    Comm const& comm, common::Span<T> data, Fn redop) {
+  auto erased = EraseType(data);
+  auto type = ToDType<T>::kType;
+
+  auto erased_fn = [type, redop](common::Span<std::int8_t const> lhs,
+                                 common::Span<std::int8_t> out) {
+    CHECK_EQ(lhs.size(), out.size()) << "Invalid input for reduction.";
+    auto lhs_t = RestoreType<T const>(lhs);
+    auto rhs_t = RestoreType<T>(out);
+    redop(lhs_t, rhs_t);
+  };
+
+  return cpu_impl::RingAllreduce(comm, erased, erased_fn, type);
+}
+}  // namespace xgboost::collective
diff --git a/src/data/array_interface.h b/src/data/array_interface.h
index c62a5cef2..0170e6a84 100644
--- a/src/data/array_interface.h
+++ b/src/data/array_interface.h
@@ -16,7 +16,7 @@
 #include <utility>
 #include <vector>
 
-#include "../common/bitfield.h"
+#include "../common/bitfield.h"  // for RBitField8
 #include "../common/common.h"
 #include "../common/error_msg.h"  // for NoF128
 #include "xgboost/base.h"
@@ -104,7 +104,20 @@ struct ArrayInterfaceErrors {
  */
 class ArrayInterfaceHandler {
  public:
-  enum Type : std::int8_t { kF2, kF4, kF8, kF16, kI1, kI2, kI4, kI8, kU1, kU2, kU4, kU8 };
+  enum Type : std::int8_t {
+    kF2 = 0,
+    kF4 = 1,
+    kF8 = 2,
+    kF16 = 3,
+    kI1 = 4,
+    kI2 = 5,
+    kI4 = 6,
+    kI8 = 7,
+    kU1 = 8,
+    kU2 = 9,
+    kU4 = 10,
+    kU8 = 11,
+  };
 
   template <typename PtrType>
   static PtrType GetPtrFromArrayData(Object::Map const &obj) {
@@ -587,6 +600,57 @@ class ArrayInterface {
   ArrayInterfaceHandler::Type type{ArrayInterfaceHandler::kF16};
 };
 
+template <typename Fn>
+auto DispatchDType(ArrayInterfaceHandler::Type dtype, Fn dispatch) {
+  switch (dtype) {
+    case ArrayInterfaceHandler::kF2: {
+#if defined(XGBOOST_USE_CUDA)
+      return dispatch(__half{});
+#else
+      LOG(FATAL) << "half type is only supported for CUDA input.";
+      break;
+#endif
+    }
+    case ArrayInterfaceHandler::kF4: {
+      return dispatch(float{});
+    }
+    case ArrayInterfaceHandler::kF8: {
+      return dispatch(double{});
+    }
+    case ArrayInterfaceHandler::kF16: {
+      using T = long double;
+      CHECK(sizeof(T) == 16) << error::NoF128();
+      return dispatch(T{});
+    }
+    case ArrayInterfaceHandler::kI1: {
+      return dispatch(std::int8_t{});
+    }
+    case ArrayInterfaceHandler::kI2: {
+      return dispatch(std::int16_t{});
+    }
+    case ArrayInterfaceHandler::kI4: {
+      return dispatch(std::int32_t{});
+    }
+    case ArrayInterfaceHandler::kI8: {
+      return dispatch(std::int64_t{});
+    }
+    case ArrayInterfaceHandler::kU1: {
+      return dispatch(std::uint8_t{});
+    }
+    case ArrayInterfaceHandler::kU2: {
+      return dispatch(std::uint16_t{});
+    }
+    case ArrayInterfaceHandler::kU4: {
+      return dispatch(std::uint32_t{});
+    }
+    case ArrayInterfaceHandler::kU8: {
+      return dispatch(std::uint64_t{});
+    }
+  }
+
+  return std::result_of_t<Fn(std::int8_t)>();
+}
+
 template <std::int32_t D, typename Fn>
 void DispatchDType(ArrayInterface<D> const array, DeviceOrd device, Fn fn) {
   // Only used for cuDF at the moment.
@@ -602,60 +666,7 @@ void DispatchDType(ArrayInterface<D> const array, DeviceOrd device, Fn fn) {
                                                       std::numeric_limits<std::size_t>::max()},
                                 array.shape, array.strides, device});
   };
-  switch (array.type) {
-    case ArrayInterfaceHandler::kF2: {
-#if defined(XGBOOST_USE_CUDA)
-      dispatch(__half{});
-#endif
-      break;
-    }
-    case ArrayInterfaceHandler::kF4: {
-      dispatch(float{});
-      break;
-    }
-    case ArrayInterfaceHandler::kF8: {
-      dispatch(double{});
-      break;
-    }
-    case ArrayInterfaceHandler::kF16: {
-      using T = long double;
-      CHECK(sizeof(long double) == 16) << error::NoF128();
-      dispatch(T{});
-      break;
-    }
-    case ArrayInterfaceHandler::kI1: {
-      dispatch(std::int8_t{});
-      break;
-    }
-    case ArrayInterfaceHandler::kI2: {
-      dispatch(std::int16_t{});
-      break;
-    }
-    case ArrayInterfaceHandler::kI4: {
-      dispatch(std::int32_t{});
-      break;
-    }
-    case ArrayInterfaceHandler::kI8: {
-      dispatch(std::int64_t{});
-      break;
-    }
-    case ArrayInterfaceHandler::kU1: {
-      dispatch(std::uint8_t{});
-      break;
-    }
-    case ArrayInterfaceHandler::kU2: {
-      dispatch(std::uint16_t{});
-      break;
-    }
-    case ArrayInterfaceHandler::kU4: {
-      dispatch(std::uint32_t{});
-      break;
-    }
-    case ArrayInterfaceHandler::kU8: {
-      dispatch(std::uint64_t{});
-      break;
-    }
-  }
+  DispatchDType(array.type, dispatch);
 }
 
 /**
diff --git a/tests/cpp/collective/test_allreduce.cc b/tests/cpp/collective/test_allreduce.cc
new file mode 100644
index 000000000..62b87e411
--- /dev/null
+++ b/tests/cpp/collective/test_allreduce.cc
@@ -0,0 +1,72 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include "../../../src/collective/allreduce.h"
+#include "../../../src/collective/tracker.h"
+#include "test_worker.h"  // for WorkerForTest, TestDistributed
+
+namespace xgboost::collective {
+
+namespace {
+class AllreduceWorker : public WorkerForTest {
+ public:
+  using WorkerForTest::WorkerForTest;
+
+  void Basic() {
+    {
+      std::vector<double> data(13, 0.0);
+      Allreduce(comm_, common::Span{data.data(), data.size()}, [](auto lhs, auto rhs) {
+        for (std::size_t i = 0; i < rhs.size(); ++i) {
+          rhs[i] += lhs[i];
+        }
+      });
+      ASSERT_EQ(std::accumulate(data.cbegin(), data.cend(), 0.0), 0.0);
+    }
+    {
+      std::vector<double> data(1, 1.0);
+      Allreduce(comm_, common::Span{data.data(), data.size()}, [](auto lhs, auto rhs) {
+        for (std::size_t i = 0; i < rhs.size(); ++i) {
+          rhs[i] += lhs[i];
+        }
+      });
+      ASSERT_EQ(data[0], static_cast<double>(comm_.World()));
+    }
+  }
+
+  void Acc() {
+    std::vector<double> data(314, 1.5);
+    Allreduce(comm_, common::Span{data.data(), data.size()}, [](auto lhs, auto rhs) {
+      for (std::size_t i = 0; i < rhs.size(); ++i) {
+        rhs[i] += lhs[i];
+      }
+    });
+    for (std::size_t i = 0; i < data.size(); ++i) {
+      auto v = data[i];
+      ASSERT_EQ(v, 1.5 * static_cast<double>(comm_.World())) << i;
+    }
+  }
+};
+
+class AllreduceTest : public SocketTest {};
+}  // namespace
+
+TEST_F(AllreduceTest, Basic) {
+  std::int32_t n_workers = std::min(7u, std::thread::hardware_concurrency());
+  TestDistributed(n_workers, [=](std::string host, std::int32_t port, std::chrono::seconds timeout,
+                                 std::int32_t r) {
+    AllreduceWorker worker{host, port, timeout, n_workers, r};
+    worker.Basic();
+  });
+}
+
+TEST_F(AllreduceTest, Sum) {
+  std::int32_t n_workers = std::min(7u, std::thread::hardware_concurrency());
+  TestDistributed(n_workers, [=](std::string host, std::int32_t port, std::chrono::seconds timeout,
+                                 std::int32_t r) {
+    AllreduceWorker worker{host, port, timeout, n_workers, r};
+    worker.Acc();
+  });
+}
+}  // namespace xgboost::collective
diff --git a/tests/cpp/collective/test_broadcast.cc b/tests/cpp/collective/test_broadcast.cc
index 485f6dcdf..0ade86567 100644
--- a/tests/cpp/collective/test_broadcast.cc
+++ b/tests/cpp/collective/test_broadcast.cc
@@ -10,8 +10,8 @@
 #include <vector>   // for vector
 
 #include "../../../src/collective/broadcast.h"  // for Broadcast
-#include "../../../src/collective/tracker.h"    // for GetHostAddress, Tracker
-#include "test_worker.h"                        // for WorkerForTest
+#include "../../../src/collective/tracker.h"    // for GetHostAddress
+#include "test_worker.h"                        // for WorkerForTest, TestDistributed
 
 namespace xgboost::collective {
 namespace {
@@ -41,28 +41,11 @@ class BroadcastTest : public SocketTest {};
 }  // namespace
 
 TEST_F(BroadcastTest, Basic) {
-  std::int32_t n_workers = std::min(24u, std::thread::hardware_concurrency());
-  std::chrono::seconds timeout{3};
-
-  std::string host;
-  ASSERT_TRUE(GetHostAddress(&host).OK());
-  RabitTracker tracker{StringView{host}, n_workers, 0, timeout};
-  auto fut = tracker.Run();
-
-  std::vector<std::thread> workers;
-  std::int32_t port = tracker.Port();
-
-  for (std::int32_t i = 0; i < n_workers; ++i) {
-    workers.emplace_back([=] {
-      Worker worker{host, port, timeout, n_workers, i};
-      worker.Run();
-    });
-  }
-
-  for (auto& t : workers) {
-    t.join();
-  }
-
-  ASSERT_TRUE(fut.get().OK());
+  std::int32_t n_workers = std::min(7u, std::thread::hardware_concurrency());
+  TestDistributed(n_workers, [=](std::string host, std::int32_t port, std::chrono::seconds timeout,
+                                 std::int32_t r) {
+    Worker worker{host, port, timeout, n_workers, r};
+    worker.Run();
+  });
 }
 }  // namespace xgboost::collective
diff --git a/tests/cpp/collective/test_worker.h b/tests/cpp/collective/test_worker.h
index 3c9d02f03..a3d6de875 100644
--- a/tests/cpp/collective/test_worker.h
+++ b/tests/cpp/collective/test_worker.h
@@ -88,4 +88,27 @@ class TrackerTest : public SocketTest {
     ASSERT_TRUE(rc.OK()) << rc.Report();
   }
 };
+
+template <typename WorkerFn>
+void TestDistributed(std::int32_t n_workers, WorkerFn worker_fn) {
+  std::chrono::seconds timeout{1};
+
+  std::string host;
+  ASSERT_TRUE(GetHostAddress(&host).OK());
+  RabitTracker tracker{StringView{host}, n_workers, 0, timeout};
+  auto fut = tracker.Run();
+
+  std::vector<std::thread> workers;
+  std::int32_t port = tracker.Port();
+
+  for (std::int32_t i = 0; i < n_workers; ++i) {
+    workers.emplace_back([=] { worker_fn(host, port, timeout, i); });
+  }
+
+  for (auto& t : workers) {
+    t.join();
+  }
+
+  ASSERT_TRUE(fut.get().OK());
+}
 }  // namespace xgboost::collective