The centre back has seven caps for Spain and made 32 appearances for Barcelona, winning two Champions League trophies and three LaLiga titles since joining in 2019. She also spent the 2021-22 season on loan at AC Milan and played 14 times.
"Laia is a world-class defender and I'm delighted to be working with her here at Arsenal," coach Jonas Eidevall said.
"She has a great mix of technical ability and physical presence and we believe this will allow her to adapt to our style here quickly.
"She has considerable experience of playing at the highest level and winning trophies, so I'm confident that she will be an excellent addition to our squad moving forward."
Arsenal, who finished third in the league last year, begin their new campaign at home to Liverpool on Oct. 1.