diff --git a/hadoop-common-project/hadoop-common/CHANGES.txt b/hadoop-common-project/hadoop-common/CHANGES.txt
index 8d952e12bd6..b915fd8f0dc 100644
--- a/hadoop-common-project/hadoop-common/CHANGES.txt
+++ b/hadoop-common-project/hadoop-common/CHANGES.txt
@@ -13,6 +13,8 @@ Trunk (unreleased changes)
     (suresh)
 
   IMPROVEMENTS
+    HADOOP-8048. Allow merging of Credentials (Daryn Sharp via tgraves)
+ 
     HADOOP-8032. mvn site:stage-deploy should be able to use the scp protocol
     to stage documents (Ravi Prakash via tgraves)
 
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Credentials.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Credentials.java
index 152a35496b8..9883604a2f9 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Credentials.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Credentials.java
@@ -230,14 +230,34 @@ public class Credentials implements Writable {
  
   /**
    * Copy all of the credentials from one credential object into another.
+   * Existing secrets and tokens are overwritten.
    * @param other the credentials to copy
    */
   public void addAll(Credentials other) {
+    addAll(other, true);
+  }
+
+  /**
+   * Copy all of the credentials from one credential object into another.
+   * Existing secrets and tokens are not overwritten.
+   * @param other the credentials to copy
+   */
+  public void mergeAll(Credentials other) {
+    addAll(other, false);
+  }
+
+  private void addAll(Credentials other, boolean overwrite) {
     for(Map.Entry<Text, byte[]> secret: other.secretKeysMap.entrySet()) {
-      secretKeysMap.put(secret.getKey(), secret.getValue());
+      Text key = secret.getKey();
+      if (!secretKeysMap.containsKey(key) || overwrite) {
+        secretKeysMap.put(key, secret.getValue());
+      }
     }
     for(Map.Entry<Text, Token<?>> token: other.tokenMap.entrySet()){
-      tokenMap.put(token.getKey(), token.getValue());
+      Text key = token.getKey();
+      if (!tokenMap.containsKey(key) || overwrite) {
+        tokenMap.put(key, token.getValue());
+      }
     }
   }
 }
diff --git a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestCredentials.java b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestCredentials.java
index 7bedd2d028a..56b5c32521d 100644
--- a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestCredentials.java
+++ b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestCredentials.java
@@ -137,4 +137,81 @@ public class TestCredentials {
     }
     tmpFileName.delete();
   }
+
+  static Text secret[] = {
+      new Text("secret1"),
+      new Text("secret2"),
+      new Text("secret3"),
+      new Text("secret4")
+  };
+  static Text service[] = {
+      new Text("service1"),
+      new Text("service2"),
+      new Text("service3"),
+      new Text("service4")
+  };
+  static Token<?> token[] = {
+      new Token<TokenIdentifier>(),
+      new Token<TokenIdentifier>(),
+      new Token<TokenIdentifier>(),
+      new Token<TokenIdentifier>()
+  };
+  
+  @Test
+  public void addAll() {
+    Credentials creds = new Credentials();
+    creds.addToken(service[0], token[0]);
+    creds.addToken(service[1], token[1]);
+    creds.addSecretKey(secret[0], secret[0].getBytes());
+    creds.addSecretKey(secret[1], secret[1].getBytes());
+
+    Credentials credsToAdd = new Credentials();
+    // one duplicate with different value, one new
+    credsToAdd.addToken(service[0], token[3]);
+    credsToAdd.addToken(service[2], token[2]);
+    credsToAdd.addSecretKey(secret[0], secret[3].getBytes());
+    credsToAdd.addSecretKey(secret[2], secret[2].getBytes());
+    
+    creds.addAll(credsToAdd);
+    assertEquals(3, creds.numberOfTokens());
+    assertEquals(3, creds.numberOfSecretKeys());
+    // existing token & secret should be overwritten
+    assertEquals(token[3], creds.getToken(service[0]));
+    assertEquals(secret[3], new Text(creds.getSecretKey(secret[0])));
+    // non-duplicate token & secret should be present
+    assertEquals(token[1], creds.getToken(service[1]));
+    assertEquals(secret[1], new Text(creds.getSecretKey(secret[1])));
+    // new token & secret should be added
+    assertEquals(token[2], creds.getToken(service[2]));
+    assertEquals(secret[2], new Text(creds.getSecretKey(secret[2])));
+  }
+
+  @Test
+  public void mergeAll() {
+    Credentials creds = new Credentials();
+    creds.addToken(service[0], token[0]);
+    creds.addToken(service[1], token[1]);
+    creds.addSecretKey(secret[0], secret[0].getBytes());
+    creds.addSecretKey(secret[1], secret[1].getBytes());
+    
+    Credentials credsToAdd = new Credentials();
+    // one duplicate with different value, one new
+    credsToAdd.addToken(service[0], token[3]);
+    credsToAdd.addToken(service[2], token[2]);
+    credsToAdd.addSecretKey(secret[0], secret[3].getBytes());
+    credsToAdd.addSecretKey(secret[2], secret[2].getBytes());
+    
+    creds.mergeAll(credsToAdd);
+    assertEquals(3, creds.numberOfTokens());
+    assertEquals(3, creds.numberOfSecretKeys());
+    // existing token & secret should not be overwritten
+    assertEquals(token[0], creds.getToken(service[0]));
+    assertEquals(secret[0], new Text(creds.getSecretKey(secret[0])));
+    // non-duplicate token & secret should be present
+    assertEquals(token[1], creds.getToken(service[1]));
+    assertEquals(secret[1], new Text(creds.getSecretKey(secret[1])));
+    // new token & secret should be added
+    assertEquals(token[2], creds.getToken(service[2]));
+    assertEquals(secret[2], new Text(creds.getSecretKey(secret[2])));
  }
+}
\ No newline at end of file