The 25-year-old had been under investigation by Saints after he posted an alleged racist tweet about double British Olympic gold medallist Mo Farah, for which he later made an apology.
But the Premiership club said on Friday that Sharman had left Franklin's Gardens because of a knee injury.
The South Africa-born player missed much of last season and has not featured since January, with Northampton believing there is "no immediate prospect" of a return to full fitness.
In an official club statement, there was no mention of Sharman's alleged tweet.
Northampton rugby director Jim Mallinder said: "It is always a shame when someone's contract comes to an end in these circumstances.
"Brett has contributed a lot to the squad in his time at Franklin's Gardens, and we wish him all the best for the future."
Sharman, quoted in the statement released by Saints, added: "I have enjoyed my time at the Saints. It is a great club with good people, and I am sure that there will be major success coming to Franklin's Gardens sooner rather than later.
"The players, management and supporters all deserve that, and I wish them every success."
Sharman's tweet was posted ahead of Farah's Olympic 5,000 metres final at London 2012 almost two weeks ago.
It was deleted shortly after it had been sent, while Sharman made a public apology and closed his Twitter account.
Farah, who added the 5,000 metres Olympic title to his 10,000 metres gold, was born in Somalia and came to live in the United Kingdom, aged eight.
In a statement released on Thursday, Northampton said: "Northampton Saints treats all such matters seriously.
"The club has a clear policy regarding social media and is following the procedures set out in that policy.
"Northampton Saints do not comment on individual cases."
Saints' announcement on Sharman is likely to be received with considerable cynicism, especially given its timing just a day after the Twitter storm blew up.
Sharman, who joined Northampton from South African side the Blue Bulls and made 68 appearances, now finds himself without a club as the new Premiership campaign prepares for its September 1 launch.
Even if he recovers from injury during the forthcoming campaign, his prospects of securing any long-term Premiership deal would seem remote at this stage, given that clubs have finalised their playing squads.