diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index a6170cd61a6..0c5df66ac2a 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -55,6 +55,9 @@ Improvements
   module into core and is now the default analyzer in
   IndexWriterConfig (Robert Muir, Mike McCandless)
 
+* LUCENE-7345: RAMDirectory now enforces write-once files as well
+  (Robert Muir, Mike McCandless)
+
 Optimizations
 
 * LUCENE-7330: Speed up conjunction queries. (Adrien Grand)
diff --git a/lucene/core/src/java/org/apache/lucene/store/RAMDirectory.java b/lucene/core/src/java/org/apache/lucene/store/RAMDirectory.java
index a4bc2eab36e..54375e66c03 100644
--- a/lucene/core/src/java/org/apache/lucene/store/RAMDirectory.java
+++ b/lucene/core/src/java/org/apache/lucene/store/RAMDirectory.java
@@ -19,6 +19,7 @@ package org.apache.lucene.store;
 
 import java.io.FileNotFoundException;
 import java.io.IOException;
+import java.nio.file.FileAlreadyExistsException;
 import java.nio.file.Files;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -177,12 +178,9 @@ public class RAMDirectory extends BaseDirectory implements Accountable {
   public IndexOutput createOutput(String name, IOContext context) throws IOException {
     ensureOpen();
     RAMFile file = newRAMFile();
-    RAMFile existing = fileMap.remove(name);
-    if (existing != null) {
-      sizeInBytes.addAndGet(-existing.sizeInBytes);
-      existing.directory = null;
+    if (fileMap.putIfAbsent(name, file) != null) {
+      throw new FileAlreadyExistsException(name);
     }
-    fileMap.put(name, file);
     return new RAMOutputStream(name, file, true);
   }
 
@@ -222,7 +220,12 @@ public class RAMDirectory extends BaseDirectory implements Accountable {
     if (file == null) {
       throw new FileNotFoundException(source);
     }
-    fileMap.put(dest, file);
+    if (fileMap.putIfAbsent(dest, file) != null) {
+      throw new FileAlreadyExistsException(dest);
+    }
+    if (!fileMap.remove(source, file)) {
+      throw new IllegalStateException("file was unexpectedly replaced: " + source);
+    }
     fileMap.remove(source);
   }
 
diff --git a/lucene/test-framework/src/java/org/apache/lucene/store/MockDirectoryWrapper.java b/lucene/test-framework/src/java/org/apache/lucene/store/MockDirectoryWrapper.java
index 209ee80005e..193d877b076 100644
--- a/lucene/test-framework/src/java/org/apache/lucene/store/MockDirectoryWrapper.java
+++ b/lucene/test-framework/src/java/org/apache/lucene/store/MockDirectoryWrapper.java
@@ -636,22 +636,6 @@ public class MockDirectoryWrapper extends BaseDirectoryWrapper {
     unSyncedFiles.add(name);
     createdFiles.add(name);
     
-    if (in instanceof RAMDirectory) {
-      RAMDirectory ramdir = (RAMDirectory) in;
-      RAMFile file = new RAMFile(ramdir);
-      RAMFile existing = ramdir.fileMap.get(name);
-    
-      // Enforce write once:
-      if (existing!=null && !name.equals("segments.gen")) {
-        throw new IOException("file " + name + " already exists");
-      } else {
-        if (existing!=null) {
-          ramdir.sizeInBytes.getAndAdd(-existing.sizeInBytes);
-          existing.directory = null;
-        }
-        ramdir.fileMap.put(name, file);
-      }
-    }
     //System.out.println(Thread.currentThread().getName() + ": MDW: create " + name);
     IndexOutput delegateOutput = in.createOutput(name, LuceneTestCase.newIOContext(randomState, context));
     final IndexOutput io = new MockIndexOutputWrapper(this, delegateOutput, name);