Mustafi had his contract terminated by mutual consent so he could secure a permanent move in January.
While he did not enjoy the best of times at the club, Mustafi had kind words for the club's supporters.
"Hi Gunners," wrote Mustafi.
"After four years, 151 matches and two FA Cup trophies, I just wanted to say thank you to the Arsenal community, our great staff, the fans and of course my team-mates.
"Together we went to through good and not so good times, but we stood together.
"Arsenal is a fantastic club with a great tradition and I always felt, that it is a pleasure and honour to wear the AFC jersey.
"My family and I will always remember the time we had in London and with Arsenal!
"Gooners, I wish you all the best! Thank you! Musti."