diff --git a/CMakeLists.txt b/CMakeLists.txt
index 109152f43..c1705f3ad 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
-project(xgboost LANGUAGES CXX C VERSION 1.5.0)
+project(xgboost LANGUAGES CXX C VERSION 1.6.0)
 include(cmake/Utils.cmake)
 list(APPEND CMAKE_MODULE_PATH "${xgboost_SOURCE_DIR}/cmake/modules")
 cmake_policy(SET CMP0022 NEW)
diff --git a/R-package/DESCRIPTION b/R-package/DESCRIPTION
index 7f0418507..b5c75fc9b 100644
--- a/R-package/DESCRIPTION
+++ b/R-package/DESCRIPTION
@@ -1,8 +1,8 @@
 Package: xgboost
 Type: Package
 Title: Extreme Gradient Boosting
-Version: 1.5.0.1
-Date: 2020-08-28
+Version: 1.6.0.1
+Date: 2021-09-25
 Authors@R: c(
   person("Tianqi", "Chen", role = c("aut"),
          email = "tianqi.tchen@gmail.com"),
diff --git a/doc/contrib/release.rst b/doc/contrib/release.rst
index ca0d1515f..4dac7718c 100644
--- a/doc/contrib/release.rst
+++ b/doc/contrib/release.rst
@@ -18,7 +18,7 @@ Making a Release
 
 1. Create an issue for the release, noting the estimated date and expected features or major fixes, pin that issue.
 2. Bump release version.
-   1. Modify ``CMakeLists.txt`` source tree, run CMake.
+   1. Modify ``CMakeLists.txt`` in source tree and ``cmake/Python_version.in`` if needed, run CMake.
    2. Modify ``DESCRIPTION`` in R-package.
    3. Run ``change_version.sh`` in ``jvm-packages/dev``
 3. Commit the change, create a PR on GitHub on release branch.  Port the bumped version to default branch, optionally with the postfix ``SNAPSHOT``.
diff --git a/include/xgboost/version_config.h b/include/xgboost/version_config.h
index c88ad615d..10aa39424 100644
--- a/include/xgboost/version_config.h
+++ b/include/xgboost/version_config.h
@@ -5,7 +5,7 @@
 #define XGBOOST_VERSION_CONFIG_H_
 
 #define XGBOOST_VER_MAJOR 1
-#define XGBOOST_VER_MINOR 5
+#define XGBOOST_VER_MINOR 6
 #define XGBOOST_VER_PATCH 0
 
 #endif  // XGBOOST_VERSION_CONFIG_H_
diff --git a/jvm-packages/pom.xml b/jvm-packages/pom.xml
index 247796663..252f3765c 100644
--- a/jvm-packages/pom.xml
+++ b/jvm-packages/pom.xml
@@ -6,7 +6,7 @@
 
     <groupId>ml.dmlc</groupId>
     <artifactId>xgboost-jvm_2.12</artifactId>
-    <version>1.5.0-SNAPSHOT</version>
+    <version>1.6.0-SNAPSHOT</version>
     <packaging>pom</packaging>
     <name>XGBoost JVM Package</name>
     <description>JVM Package for XGBoost</description>
diff --git a/jvm-packages/xgboost4j-example/pom.xml b/jvm-packages/xgboost4j-example/pom.xml
index 98ce4e948..da6ef2be6 100644
--- a/jvm-packages/xgboost4j-example/pom.xml
+++ b/jvm-packages/xgboost4j-example/pom.xml
@@ -6,10 +6,10 @@
     <parent>
         <groupId>ml.dmlc</groupId>
         <artifactId>xgboost-jvm_2.12</artifactId>
-        <version>1.5.0-SNAPSHOT</version>
+        <version>1.6.0-SNAPSHOT</version>
     </parent>
     <artifactId>xgboost4j-example_2.12</artifactId>
-    <version>1.5.0-SNAPSHOT</version>
+    <version>1.6.0-SNAPSHOT</version>
     <packaging>jar</packaging>
     <build>
         <plugins>
@@ -26,7 +26,7 @@
         <dependency>
             <groupId>ml.dmlc</groupId>
             <artifactId>xgboost4j-spark_${scala.binary.version}</artifactId>
-            <version>1.5.0-SNAPSHOT</version>
+            <version>1.6.0-SNAPSHOT</version>
         </dependency>
         <dependency>
             <groupId>org.apache.spark</groupId>
@@ -37,7 +37,7 @@
         <dependency>
             <groupId>ml.dmlc</groupId>
             <artifactId>xgboost4j-flink_${scala.binary.version}</artifactId>
-            <version>1.5.0-SNAPSHOT</version>
+            <version>1.6.0-SNAPSHOT</version>
         </dependency>
         <dependency>
             <groupId>org.apache.commons</groupId>
diff --git a/jvm-packages/xgboost4j-flink/pom.xml b/jvm-packages/xgboost4j-flink/pom.xml
index 11ad0d273..d02d1eacb 100644
--- a/jvm-packages/xgboost4j-flink/pom.xml
+++ b/jvm-packages/xgboost4j-flink/pom.xml
@@ -6,10 +6,10 @@
     <parent>
         <groupId>ml.dmlc</groupId>
         <artifactId>xgboost-jvm_2.12</artifactId>
-        <version>1.5.0-SNAPSHOT</version>
+        <version>1.6.0-SNAPSHOT</version>
     </parent>
     <artifactId>xgboost4j-flink_2.12</artifactId>
-    <version>1.5.0-SNAPSHOT</version>
+    <version>1.6.0-SNAPSHOT</version>
     <build>
         <plugins>
             <plugin>
@@ -26,7 +26,7 @@
         <dependency>
             <groupId>ml.dmlc</groupId>
             <artifactId>xgboost4j_${scala.binary.version}</artifactId>
-            <version>1.5.0-SNAPSHOT</version>
+            <version>1.6.0-SNAPSHOT</version>
         </dependency>
         <dependency>
             <groupId>org.apache.commons</groupId>
diff --git a/jvm-packages/xgboost4j-gpu/pom.xml b/jvm-packages/xgboost4j-gpu/pom.xml
index 6c2e22b7d..5d59a42a9 100644
--- a/jvm-packages/xgboost4j-gpu/pom.xml
+++ b/jvm-packages/xgboost4j-gpu/pom.xml
@@ -6,10 +6,10 @@
     <parent>
         <groupId>ml.dmlc</groupId>
         <artifactId>xgboost-jvm_2.12</artifactId>
-        <version>1.5.0-SNAPSHOT</version>
+        <version>1.6.0-SNAPSHOT</version>
     </parent>
     <artifactId>xgboost4j-gpu_2.12</artifactId>
-    <version>1.5.0-SNAPSHOT</version>
+    <version>1.6.0-SNAPSHOT</version>
     <packaging>jar</packaging>
 
     <properties>
diff --git a/jvm-packages/xgboost4j-spark-gpu/pom.xml b/jvm-packages/xgboost4j-spark-gpu/pom.xml
index 6af0d196c..0cd268770 100644
--- a/jvm-packages/xgboost4j-spark-gpu/pom.xml
+++ b/jvm-packages/xgboost4j-spark-gpu/pom.xml
@@ -6,7 +6,7 @@
     <parent>
         <groupId>ml.dmlc</groupId>
         <artifactId>xgboost-jvm_2.12</artifactId>
-        <version>1.5.0-SNAPSHOT</version>
+        <version>1.6.0-SNAPSHOT</version>
     </parent>
     <artifactId>xgboost4j-spark-gpu_2.12</artifactId>
     <build>
@@ -24,7 +24,7 @@
         <dependency>
             <groupId>ml.dmlc</groupId>
             <artifactId>xgboost4j-gpu_${scala.binary.version}</artifactId>
-            <version>1.5.0-SNAPSHOT</version>
+            <version>1.6.0-SNAPSHOT</version>
         </dependency>
         <dependency>
             <groupId>org.apache.spark</groupId>
diff --git a/jvm-packages/xgboost4j-spark/pom.xml b/jvm-packages/xgboost4j-spark/pom.xml
index 771b126b2..3fe38f875 100644
--- a/jvm-packages/xgboost4j-spark/pom.xml
+++ b/jvm-packages/xgboost4j-spark/pom.xml
@@ -6,7 +6,7 @@
     <parent>
         <groupId>ml.dmlc</groupId>
         <artifactId>xgboost-jvm_2.12</artifactId>
-        <version>1.5.0-SNAPSHOT</version>
+        <version>1.6.0-SNAPSHOT</version>
     </parent>
     <artifactId>xgboost4j-spark_2.12</artifactId>
     <build>
@@ -24,7 +24,7 @@
         <dependency>
             <groupId>ml.dmlc</groupId>
             <artifactId>xgboost4j_${scala.binary.version}</artifactId>
-            <version>1.5.0-SNAPSHOT</version>
+            <version>1.6.0-SNAPSHOT</version>
         </dependency>
         <dependency>
             <groupId>org.apache.spark</groupId>
diff --git a/jvm-packages/xgboost4j/pom.xml b/jvm-packages/xgboost4j/pom.xml
index eaa94ae62..c911eaa6e 100644
--- a/jvm-packages/xgboost4j/pom.xml
+++ b/jvm-packages/xgboost4j/pom.xml
@@ -6,10 +6,10 @@
     <parent>
         <groupId>ml.dmlc</groupId>
         <artifactId>xgboost-jvm_2.12</artifactId>
-        <version>1.5.0-SNAPSHOT</version>
+        <version>1.6.0-SNAPSHOT</version>
     </parent>
     <artifactId>xgboost4j_2.12</artifactId>
-    <version>1.5.0-SNAPSHOT</version>
+    <version>1.6.0-SNAPSHOT</version>
     <packaging>jar</packaging>
 
     <dependencies>
diff --git a/python-package/xgboost/VERSION b/python-package/xgboost/VERSION
index 59b9db0c7..bcfbf475d 100644
--- a/python-package/xgboost/VERSION
+++ b/python-package/xgboost/VERSION
@@ -1 +1 @@
-1.5.0-dev
+1.6.0-dev