Is Objective-C’s NSMutableArray thread-safe?

No.

It is not thread safe and if you need to modify your mutable array from another thread you should use NSLock to ensure everything goes as planned:

NSLock *arrayLock = [[NSLock alloc] init];

[...] 

[arrayLock lock]; // NSMutableArray isn't thread-safe
[myMutableArray addObject:@"something"];
[myMutableArray removeObjectAtIndex:5];
[arrayLock unlock];