Keane, 24, has won 10 caps for Republic of Ireland at Under-19s level, and becomes manager Tom Shaw's latest recruit at the Holy Blues.
The left-back has made more than 100 career appearances, appearing for Basford, Alfreton, Boston United and Kidderminster Harriers. His most notable spells though were at Nuneaton Town and Worcester City, where he faced off against Gainsborough Trinity in the National League North.
He said: "I really am excited to be a part of what is being put together here at Gainsborough. It is clear to see he is assembling a young, high energy side together that could well surprise a few.
"Talking to the gaffer, his vision suits how I want to play the game and he is really focussed on improving the players and getting the best out of them. I'm always looking to improve, so being part of things this season look great. The way everything is set up, looks set up to be successful and I can't wait to get started."
The club also recently announced the signings of Joe Wilson, Toby Gould, Jack Gibson and Gianluca Bucci, while Neal Bishop has joined the club in a player-coaching role.
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