Midfielder Harry, 25, joins from Boston United and previously played more than 50 games in the Football League for Doncaster.
Manager Martin McIntosh said: “On paper Harry may not seem like a high-profile signing but when you look into his background, speak to people and see that he’s played for Doncaster in the Football League, it’s clear he has lots of ability.
“He has been quite unlucky over recent years, but I’ve spoken to him and he is determined to show his talent.
“I know he has been working really hard in this off season and if he hits the ground running early on, he is going to be a really good addition to the squad for us. We're delighted to have him onboard.”
Middleton has had spells with Gainsborough and Cleethorpes Town in recent seasons.
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