Wigan Athletic Academy product joins League One rival despite initial hopes of a return
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The sale of the 21-year-old - who represented both Scotland and England at junior level - in the summer of 2020 helped to keep Latics afloat, after the club was placed into administration.
Weir has spent the last couple of years on loan in League One, at Cambridge and then Morecambe.
Indeed, it's understood Latics were very interested in the possibility of bringing him back to the DW for another season-long loan, only to be told at the start of summer he was earmarked for a move to the Championship.
However, after Latics switched their transfer sights, Weir is back in League One, with the Seasiders winning the race for his signature.
"I'm ready to get going and can't wait to get started," said Weir, who remains Latics' youngest ever player, having turned out for them against Accrington Stanley in the Football League Trophy at the age of 15.
"I'm a northern boy and know the club from afar.
"Having spoken to the head coach here I can tell we are aligned in how we like football to be played.
"I'm hoping I can perform and contribute to the team as much as possible this season, and we can have a successful campaign."
"We feel he has excellent midfield attributes in and out of possession that will prove to be invaluable this season
"We are delighted he has chosen us over other options he had, as we look to help him develop in the next step of his career, and would like to thank Brighton for entrusting us in his development.
"We look forward to helping him improve his game this season."
One of Weir’s team-mates at Blackpool will be fellow Latics Academy product Kyle Joseph, who joined the Seasiders from Swansea City earlier this summer.
Joseph, a Scotland junior international, helped Latics stay in League One against the odds in 2020/21 before being snapped up by the Swans.