diff --git a/CHANGES.txt b/CHANGES.txt
index daae8395e0..30d111e04a 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 4.0-alpha5
+ * Fail startup if -Xmn is set when the G1 garbage collector is used (CASSANDRA-15839)
  * generateSplits method replaced the generateRandomTokens for ReplicationAwareTokenAllocator. (CASSANDRA-15877)
  * Several mbeans are not unregistered when dropping a keyspace and table (CASSANDRA-14888)
  * Update defaults for server and client TLS settings (CASSANDRA-15262)
diff --git a/conf/cassandra-env.sh b/conf/cassandra-env.sh
index 724498a4e9..10fbf31211 100644
--- a/conf/cassandra-env.sh
+++ b/conf/cassandra-env.sh
@@ -176,6 +176,13 @@ elif [ $DEFINED_XMN -ne 0 ] && [ $USING_CMS -eq 0 ]; then
     JVM_OPTS="$JVM_OPTS -Xmn${HEAP_NEWSIZE}"
 fi
 
+# We fail to start if -Xmn is used with G1 GC is being used
+# See comments for -Xmn in jvm-server.options
+if [ $DEFINED_XMN -eq 0 ] && [ $USING_G1 -eq 0 ]; then
+    echo "It is not recommended to set -Xmn with the G1 garbage collector. See comments for -Xmn in jvm-server.options for details."
+    exit 1
+fi
+
 if [ "$JVM_ARCH" = "64-Bit" ] && [ $USING_CMS -eq 0 ]; then
     JVM_OPTS="$JVM_OPTS -XX:+UseCondCardMark"
 fi