Cooper traveled to Plymouth for talks with Ryan Lowe last night and was in Home Park today to finalize his move and pose for a photo wearing his number 32 jersey.
It was the number Cooper wore during his successful loan with the Pilgrims during the shortened 2019/20 COVID-19 season.
Cooper was made available for a transfer by Peterborough last month after signing midfielder Ryan Broom from Cheltenham Town for an undisclosed amount and had to cut his payroll to comply with new EFL salary cap rules .
Now, after a summer of haggling between the two clubs, a deal has been struck to bring the 23-year-old back to Home Park.
Cooper could return to the game for Argyle against Championship club Queens Park Rangers in the Carabao Cup first round at Home Park on Saturday.
He gives Lowe the option of playing him as a central midfielder or as a left-back, which was an area of his team that he was looking to strengthen after Callum McFadzean left.
Cooper told the Argyle club’s official website: “I’ve always wanted to come back, from what we did last season, I thought we would have kept winning (League Two) anyway. I hope we can continue this season.
“I have makeup on, it’s everywhere now. Over the past week we’ve had discussions, everything just moved quickly, and I couldn’t be happier to be here.
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Cooper will have to wait until Tuesday, November 24 for the chance to play for Argyle against Peterborough for the first time.
It will be then that the Pilgrims travel to London Road to take on the side of Darren Ferguson, who appears ready for promotion in 2020/21.
The second leg between the teams will take place at Home Park on Tuesday 23 February.