The 25-year-old Chile international completed his medical in London on Thursday, and will join up with his new team-mates at the end of July after spending some time on holiday following the World Cup.
Sanchez, who won the 2013 Primera Division title and 2012 Copa del Rey with Barca, is likely to make his first appearance for his new club at the Emirates Cup over the weekend of August 2-3.
The fee is believed to be around £30m.
The Gunners are also expected to bring in France defender Mathieu Debuchy from Newcastle, as manager Arsene Wenger looks to build on last season's FA Cup final success.
Arsenal manager Arsène Wenger said: “As many people have seen during the World Cup this summer, Alexis is a fantastic footballer and we are delighted that he is joining us at Arsenal Football Club.
“Alexis will add power, creativity and much quality to our squad and we are all looking forward to him joining up with us in a few weeks. He has consistently produced top-quality performances at the highest level for a number of seasons now and we are all excited to see him integrate into the Arsenal squad. I’m sure all Arsenal supporters will join me in welcoming Alexis to our club.”
Sanchez said: “I’m so happy to be joining Arsenal, a club which has a great manager, a fantastic squad of players, huge support around the world and a great stadium in London. I’m looking forward to meeting my new team-mates and to be playing for Arsenal in the Premier League and Champions League. I will give my best to Arsenal and want to make all the supporters happy.”
VIEW FROM SPAIN
Gonzalo Vazquez (Eurosport Spain): Alexis Sanchez has probably had more critics at Barcelona than he deserved but was a bit inconsistent. Sometimes he would miss clear chances but he would mix that with some brilliant moments. I think it is decent business for Barcelona and he won't be missed as Luis Suarez looks certain to sign. But Sanchez and Arsenal looks like a good fit. He has a good personality; he can play either as a striker or a winger, and is the type of guy who would fit in well with any team.
- Sports & Recreation
- Alexis Sanchez
- Arsene Wenger