The Saints sacked Martin after their 5-0 loss to Tottenham, which saw them concede four in the first 25 minutes.
They were already in the relegation zone and look like near certainties to go down.
Post-game, Harwood-Bellis went onto Instagram and posted: "Thank you, gaffer. You gave me the opportunity to grow wearing the shirt & i’ll forever hold onto what we did last season. Best of luck with the future!"
The Saints are now pushing to hire a new manager, with the likes of Kasper Hjulmand, David Moyes, Graham Potter, and Carlos Corberan all linked.
- Get breaking football news faster with the new Tribal Football app! Download now on iOS and Android to stay ahead of all the latest transfers and football updates. Download now: App Store, Google Play