Middleton, 25, re-unites with Gingerbreads boss Martin McIntosh, who he had a loan spell under at Buxton.
He said: “I am really happy to be here.
“It’s a new challenge for me coming to play here and it’s one that I am really excited by and am looking forward to getting started as soon as we can.
“I’ve played against Grantham a couple times before in pre-season friendlies and have worked under the manager and Tim Ryan during my time on loan at Buxton and to work with them again was a no-brainer.
“When I was told about the plans and ambitions the club had by both the manager and the chairman it really appealed to me.
“I didn’t have the best of years last season personally, as I didn’t play as much as I would have liked to and the opportunity to hopefully impress and get regular games made my choice even easier.
“I have a lot of trust towards the management and want to help the team as much as I can, and finish as high as possible up the table.”
Meanwhile, boss McIntosh has confirmed that experienced Shane Clarke has committed to the club for 2020/21.
Clarke, 32, was McIntosh’s first signing when he took over two games before the season was ended prematurely.
He joins experienced goalkeeper Jan Budtz and midfielder Alistair Taylor, who joined from Rossington Main and Buxton respectively this week, in the squad.
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