diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 08e15207a43..129de06b48e 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -1147,7 +1147,7 @@ class IMAP4_SSL(IMAP4):
         chunks = []
         read = 0
         while read < size:
-            data = self.sslobj.read(size-read)
+            data = self.sslobj.read(min(size-read, 16384))
             read += len(data)
             chunks.append(data)
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 4d40719e63a..0926e7f0090 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -15,6 +15,9 @@ Core and builtins
 Library
 -------
 
+- Bug #1389051: imaplib causes excessive memory fragmentation when reading
+  large messages.
+
 - Bug #1433694: minidom's .normalize() failed to set .nextSibling for
   last child element.