Arsenal sign Women's World Cup winner Codina from Barcelona
Codina started all four of Spain's World Cup knock-out stage matches, culminating with their final victory over England earlier in August.
The 23-year-old centre-back has been capped seven times by Spain and made 32 appearances for Barcelona, winning three La Liga titles and two Champions League crowns.
"Laia is a world-class defender and I'm delighted to be working with her here at Arsenal," Gunners head 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."
Codina added: "I knew all about the history of this club, so when I found out Arsenal were interested, I didn't think twice.
"We want to compete in all four competitions this season and I'm looking forward to making some memories with our supporters here."
