Fletcher had spent time on loan at Plymouth last season.
Argyle boss Paul Sturrock said: "Carl has great leadership qualities. He leads by example and has been the captain of Wales.
"I am delighted to have him at the club and look forward to him being an ever-present in the team next season.
"I tried to get him at the beginning of last season. He influenced the club when he came in February and, if I had managed to get him last summer, we would have benefited from that influence."