diff --git a/guava-tests/test/com/google/common/io/BaseEncodingTest.java b/guava-tests/test/com/google/common/io/BaseEncodingTest.java
index 82c1b4e1994c..15cf4e11e1ad 100644
--- a/guava-tests/test/com/google/common/io/BaseEncodingTest.java
+++ b/guava-tests/test/com/google/common/io/BaseEncodingTest.java
@@ -572,6 +572,34 @@ private static void testStreamingDecodes(BaseEncoding encoding, String encoded,
     }
   }
 
+  @GwtIncompatible // Writer,OutputStream
+  public void testEncodingStreamCloseIsIdempotent() throws IOException {
+    StringWriter writer = new StringWriter();
+    OutputStream encodingStream = base64().encodingStream(writer);
+    encodingStream.write(0);
+    encodingStream.close();
+    assertThat(writer.toString()).isEqualTo("AA==");
+    // Closing again should have no effect.
+    encodingStream.close();
+    assertThat(writer.toString()).isEqualTo("AA==");
+  }
+
+  @GwtIncompatible // Writer,OutputStream
+  public void testEncodingStreamWriteAfterClose() throws IOException {
+    StringWriter writer = new StringWriter();
+    OutputStream encodingStream = base64().encodingStream(writer);
+    encodingStream.close();
+    assertThrows(IOException.class, () -> encodingStream.write(0));
+  }
+
+  @GwtIncompatible // Writer,OutputStream
+  public void testEncodingStreamFlushAfterClose() throws IOException {
+    StringWriter writer = new StringWriter();
+    OutputStream encodingStream = base64().encodingStream(writer);
+    encodingStream.close();
+    assertThrows(IOException.class, () -> encodingStream.flush());
+  }
+
   public void testToString() {
     assertThat(base64().toString()).isEqualTo("BaseEncoding.base64().withPadChar('=')");
     assertThat(base32Hex().omitPadding().toString())
diff --git a/guava-tests/test/com/google/common/io/ReflectionFreeAssertThrows.java b/guava-tests/test/com/google/common/io/ReflectionFreeAssertThrows.java
index 957bfb4902a1..fc391c54f8e7 100644
--- a/guava-tests/test/com/google/common/io/ReflectionFreeAssertThrows.java
+++ b/guava-tests/test/com/google/common/io/ReflectionFreeAssertThrows.java
@@ -23,6 +23,7 @@
 import com.google.common.base.VerifyException;
 import com.google.common.collect.ImmutableMap;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import java.io.IOException;
 import java.lang.reflect.InvocationTargetException;
 import java.nio.charset.UnsupportedCharsetException;
 import java.util.ConcurrentModificationException;
@@ -136,6 +137,7 @@ ImmutableMap<Class<? extends Throwable>, Predicate<Throwable>> exceptions() {
           .put(ExecutionException.class, e -> e instanceof ExecutionException)
           .put(IllegalArgumentException.class, e -> e instanceof IllegalArgumentException)
           .put(IllegalStateException.class, e -> e instanceof IllegalStateException)
+          .put(IOException.class, e -> e instanceof IOException)
           .put(IndexOutOfBoundsException.class, e -> e instanceof IndexOutOfBoundsException)
           .put(NoSuchElementException.class, e -> e instanceof NoSuchElementException)
           .put(NullPointerException.class, e -> e instanceof NullPointerException)
diff --git a/guava/src/com/google/common/io/BaseEncoding.java b/guava/src/com/google/common/io/BaseEncoding.java
index a12d7519f55f..6b3f80aef4e5 100644
--- a/guava/src/com/google/common/io/BaseEncoding.java
+++ b/guava/src/com/google/common/io/BaseEncoding.java
@@ -645,9 +645,13 @@ public OutputStream encodingStream(Writer out) {
         int bitBuffer = 0;
         int bitBufferLength = 0;
         int writtenChars = 0;
+        boolean closed = false;
 
         @Override
-        public void write(int b) throws IOException {
+        public synchronized void write(int b) throws IOException {
+          if (closed) {
+            throw new IOException("Stream is closed");
+          }
           bitBuffer <<= 8;
           bitBuffer |= b & 0xFF;
           bitBufferLength += 8;
@@ -660,12 +664,19 @@ public void write(int b) throws IOException {
         }
 
         @Override
-        public void flush() throws IOException {
+        public synchronized void flush() throws IOException {
+          if (closed) {
+            throw new IOException("Stream is closed");
+          }
           out.flush();
         }
 
         @Override
-        public void close() throws IOException {
+        public synchronized void close() throws IOException {
+          if (closed) {
+            return;
+          }
+          closed = true;
           if (bitBufferLength > 0) {
             int charIndex = (bitBuffer << (alphabet.bitsPerChar - bitBufferLength)) & alphabet.mask;
             out.write(alphabet.encode(charIndex));