The ex-England Under-21s midfielder has been loaned out three times after arriving from Exeter almost four years ago.
But under Graham Potter he is in his longest run of starts at the club, with his current contract expiring at the end of the season and talks set to start over a fresh deal.
Grimes, 23, was wanted by Liverpool ahead of his move to Swansea but was loaned out to Blackburn, Leeds and Northampton.
He was playing left-back at the start of the season before seizing his chance in midfield and impressing Potter.
The Swans chief said: "He is a guy who worked incredibly hard during pre-season, really put a big effort in. He is enjoying his football, he is one of those guys who has really dug in and helped us.
"He has filled in at left-back when we have needed and now into midfield and he has got on with. Anything you ask him to do, he does.
"He is a tremendous character and I am really pleased he is in the team and helping us.
"He is a team player, he will do what is required and guys like him are good people to have around.
"You can sense he has had some time away, he has had to fight for this opportunity and the bottom line is he deserves to play.
"He does everything to the best of his ability and I am pleased for him."
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