diff --git a/CHANGES.txt b/CHANGES.txt
index ed52a77068..893dd746ed 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 4.0
+ * Output warning if user increases RF (CASSANDRA-13079)
  * Remove pre-3.0 streaming compatibility code for 4.0 (CASSANDRA-13081)
  * Add support for + and - operations on dates (CASSANDRA-11936)
  * Fix consistency of incrementally repaired data (CASSANDRA-9143)
diff --git a/doc/source/faq/index.rst b/doc/source/faq/index.rst
index 24575a47ee..acb7538d68 100644
--- a/doc/source/faq/index.rst
+++ b/doc/source/faq/index.rst
@@ -98,15 +98,16 @@ token on the next restart.
 Can I change the replication factor (a a keyspace) on a live cluster?
 ---------------------------------------------------------------------
 
-Yes, but it will require running repair (or cleanup) to change the replica count of existing data:
+Yes, but it will require running a full repair (or cleanup) to change the replica count of existing data:
 
 - :ref:`Alter <alter-keyspace-statement>` the replication factor for desired keyspace (using cqlsh for instance).
 - If you're reducing the replication factor, run ``nodetool cleanup`` on the cluster to remove surplus replicated data.
   Cleanup runs on a per-node basis.
-- If you're increasing the replication factor, run ``nodetool repair`` to ensure data is replicated according to the new
+- If you're increasing the replication factor, run ``nodetool repair -full`` to ensure data is replicated according to the new
   configuration. Repair runs on a per-replica set basis. This is an intensive process that may result in adverse cluster
   performance. It's highly recommended to do rolling repairs, as an attempt to repair the entire cluster at once will
-  most likely swamp it.
+  most likely swamp it. Note that you will need to run a full repair (``-full``) to make sure that already repaired
+  sstables are not skipped.
 
 .. _can-large-blob:
 
diff --git a/src/java/org/apache/cassandra/cql3/statements/AlterKeyspaceStatement.java b/src/java/org/apache/cassandra/cql3/statements/AlterKeyspaceStatement.java
index 68700f2a6c..0de5b2a6f0 100644
--- a/src/java/org/apache/cassandra/cql3/statements/AlterKeyspaceStatement.java
+++ b/src/java/org/apache/cassandra/cql3/statements/AlterKeyspaceStatement.java
@@ -18,7 +18,9 @@
 package org.apache.cassandra.cql3.statements;
 
 import org.apache.cassandra.auth.Permission;
+import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.exceptions.*;
+import org.apache.cassandra.locator.AbstractReplicationStrategy;
 import org.apache.cassandra.locator.LocalStrategy;
 import org.apache.cassandra.schema.KeyspaceMetadata;
 import org.apache.cassandra.schema.KeyspaceParams;
@@ -26,7 +28,9 @@ import org.apache.cassandra.schema.MigrationManager;
 import org.apache.cassandra.schema.Schema;
 import org.apache.cassandra.schema.SchemaConstants;
 import org.apache.cassandra.service.ClientState;
+import org.apache.cassandra.service.ClientWarn;
 import org.apache.cassandra.service.QueryState;
+import org.apache.cassandra.service.StorageService;
 import org.apache.cassandra.transport.Event;
 
 public class AlterKeyspaceStatement extends SchemaAlteringStatement
@@ -74,9 +78,26 @@ public class AlterKeyspaceStatement extends SchemaAlteringStatement
             params.validate(name);
             if (params.replication.klass.equals(LocalStrategy.class))
                 throw new ConfigurationException("Unable to use given strategy class: LocalStrategy is reserved for internal use.");
+            warnIfIncreasingRF(ksm, params);
         }
     }
 
+    private void warnIfIncreasingRF(KeyspaceMetadata ksm, KeyspaceParams params)
+    {
+        AbstractReplicationStrategy oldStrategy = AbstractReplicationStrategy.createReplicationStrategy(ksm.name,
+                                                                                                        ksm.params.replication.klass,
+                                                                                                        StorageService.instance.getTokenMetadata(),
+                                                                                                        DatabaseDescriptor.getEndpointSnitch(),
+                                                                                                        ksm.params.replication.options);
+        AbstractReplicationStrategy newStrategy = AbstractReplicationStrategy.createReplicationStrategy(keyspace(),
+                                                                                                        params.replication.klass,
+                                                                                                        StorageService.instance.getTokenMetadata(),
+                                                                                                        DatabaseDescriptor.getEndpointSnitch(),
+                                                                                                        params.replication.options);
+        if (newStrategy.getReplicationFactor() > oldStrategy.getReplicationFactor())
+            ClientWarn.instance.warn("When increasing replication factor you need to run a full (-full) repair to distribute the data.");
+    }
+
     public Event.SchemaChange announceMigration(QueryState queryState, boolean isLocalOnly) throws RequestValidationException
     {
         KeyspaceMetadata oldKsm = Schema.instance.getKeyspaceMetadata(name);