diff --git a/Signal/src/AppDelegate.m b/Signal/src/AppDelegate.m
index c626266ba..26cbab7ae 100644
--- a/Signal/src/AppDelegate.m
+++ b/Signal/src/AppDelegate.m
@@ -636,6 +636,9 @@ static NSTimeInterval launchStartedAt;
     // be called _before_ we become active.
     [self clearAllNotificationsAndRestoreBadgeCount];
 
+    // On every activation, clear old temp directories.
+    ClearOldTemporaryDirectories();
+
     OWSLogInfo(@"applicationDidBecomeActive completed.");
 }
 
diff --git a/SignalServiceKit/src/Util/OWSFileSystem.h b/SignalServiceKit/src/Util/OWSFileSystem.h
index 0bbad4c92..aa4dcc90d 100644
--- a/SignalServiceKit/src/Util/OWSFileSystem.h
+++ b/SignalServiceKit/src/Util/OWSFileSystem.h
@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
 //
 
 NS_ASSUME_NONNULL_BEGIN
@@ -9,6 +9,7 @@ NS_ASSUME_NONNULL_BEGIN
 // unless the temp data may need to be accessed while the device is locked.
 NSString *OWSTemporaryDirectory(void);
 NSString *OWSTemporaryDirectoryAccessibleAfterFirstAuth(void);
+void ClearOldTemporaryDirectories(void);
 
 @interface OWSFileSystem : NSObject
 
diff --git a/SignalServiceKit/src/Util/OWSFileSystem.m b/SignalServiceKit/src/Util/OWSFileSystem.m
index 183e9edb6..36d4a8f07 100644
--- a/SignalServiceKit/src/Util/OWSFileSystem.m
+++ b/SignalServiceKit/src/Util/OWSFileSystem.m
@@ -1,10 +1,11 @@
 //
-//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
 //
 
 #import "OWSFileSystem.h"
 #import "OWSError.h"
 #import "TSConstants.h"
+#import <SignalCoreKit/NSDate+OWS.h>
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -336,12 +337,26 @@ NS_ASSUME_NONNULL_BEGIN
 
 @end
 
+#pragma mark -
+
 NSString *OWSTemporaryDirectory(void)
 {
-    NSString *dirName = @"ows_temp";
-    NSString *dirPath = [NSTemporaryDirectory() stringByAppendingPathComponent:dirName];
-    BOOL success = [OWSFileSystem ensureDirectoryExists:dirPath fileProtectionType:NSFileProtectionComplete];
-    OWSCAssert(success);
+    static NSString *dirPath;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        NSString *dirName = [NSString stringWithFormat:@"ows_temp_%@", NSUUID.UUID.UUIDString];
+        dirPath = [NSTemporaryDirectory() stringByAppendingPathComponent:dirName];
+        BOOL success = [OWSFileSystem ensureDirectoryExists:dirPath fileProtectionType:NSFileProtectionComplete];
+        OWSCAssert(success);
+
+        // On launch, clear old temp directories.
+        //
+        // NOTE: ClearOldTemporaryDirectoriesSync() will call this function
+        // OWSTemporaryDirectory(), but there's no risk of deadlock;
+        // ClearOldTemporaryDirectories() calls ClearOldTemporaryDirectoriesSync()
+        // after a long delay.
+        ClearOldTemporaryDirectories();
+    });
     return dirPath;
 }
 
@@ -354,4 +369,70 @@ NSString *OWSTemporaryDirectoryAccessibleAfterFirstAuth(void)
     return dirPath;
 }
 
+void ClearOldTemporaryDirectoriesSync(void)
+{
+    // Ignore the "current" temp directory.
+    NSString *currentTempDirName = OWSTemporaryDirectory().lastPathComponent;
+
+    NSDate *thresholdDate = CurrentAppContext().appLaunchTime;
+    NSString *dirPath = NSTemporaryDirectory();
+    NSError *error;
+    NSArray<NSString *> *fileNames = [[NSFileManager defaultManager] contentsOfDirectoryAtPath:dirPath error:&error];
+    if (error) {
+        OWSCFailDebug(@"contentsOfDirectoryAtPath error: %@", error);
+        return;
+    }
+    for (NSString *fileName in fileNames) {
+        if (!CurrentAppContext().isAppForegroundAndActive) {
+            // Abort if app not active.
+            return;
+        }
+        if ([fileName isEqualToString:currentTempDirName]) {
+            continue;
+        }
+
+        NSString *filePath = [dirPath stringByAppendingPathComponent:fileName];
+
+        // Delete files with either:
+        //
+        // a) "ows_temp" name prefix.
+        // b) modified time before app launch time.
+        if (![fileName hasPrefix:@"ows_temp"]) {
+            NSError *error;
+            NSDictionary *attributes = [[NSFileManager defaultManager] attributesOfItemAtPath:filePath error:&error];
+            if (!attributes || error) {
+                // This is fine; the file may have been deleted since we found it.
+                OWSLogError(@"Could not get attributes of file or directory at: %@", filePath);
+                continue;
+            }
+            // Don't delete files which were created in the last N minutes.
+            NSDate *creationDate = attributes.fileModificationDate;
+            if ([creationDate isAfterDate:thresholdDate]) {
+                OWSLogInfo(@"Skipping file due to age: %f", fabs([creationDate timeIntervalSinceNow]));
+                continue;
+            }
+        }
+
+        OWSLogVerbose(@"Removing temp file or directory: %@", filePath);
+        if (![OWSFileSystem deleteFile:filePath]) {
+            // This can happen if the app launches before the phone is unlocked.
+            // Clean up will occur when app becomes active.
+            OWSLogWarn(@"Could not delete old temp directory: %@", filePath);
+        }
+    }
+}
+
+// NOTE: We need to call this method on launch _and_ every time the app becomes active,
+//       since file protection may prevent it from succeeding in the background.
+void ClearOldTemporaryDirectories(void)
+{
+    // We use the lowest priority queue for this, and wait N seconds
+    // to avoid interfering with app startup.
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.f * NSEC_PER_SEC)),
+        dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0),
+        ^{
+            ClearOldTemporaryDirectoriesSync();
+        });
+}
+
 NS_ASSUME_NONNULL_END