The Swedish under-21 international joins the Scottish side as he tries to earn minutes on the pitch and progress his career.
Under-21s head coach Shannon Ruth spoke about what an opportunity it is for the defender.
“Casper has been a part of the academy set-up for four years now, so this is a great opportunity for him to take his first step into a men's first team environment.”
Thistle manager Kris Doolan said:
“We’ve had Casper in training with us for the past few days and we’ve been extremely impressed with what we’ve seen. I know Gordon Greer and Gary Dicker who both work in the academy set-up at Brighton and they both think very highly of Casper.”
“On the right-hand side of the defence, he’ll bring pace and energy and a real attacking intent which I think will really fit into our style of play. He’s already fit in well with boys on the training ground and we’re excited to see what he’ll bring to the team.”
Nilsson is one of several Brighon players who either have gone on loan or are rumoured to including Facundo Buonanotte, Ibrahim Osman, Andrew Moran, Carl Rushworth and Mark O’Mahony.