Suarez will link up again with Arsenal boss Unai Emery, with whom he worked during a loan spell at Sevilla in the 2014-15 season.
The 25-year-old has played 71 games for Barcelona, scoring eight goals and once for Spain.
“We are very happy that Denis Suarez is joining us,” Emery said on Arsenal’s website. “He is a player we know well and I have worked with him at Sevilla.
“He brings us quality and options in many different attacking positions, so he’ll be able to help the team.”
The Spanish playmaker posted a picture of himself with Barcelona teammates on his personal Twitter account and said: “You’re a group of amazing people! Thank you for everything, see you soon. Good luck and Forca Barca.”
Arsenal are fourth in the Premier League after beating Cardiff 2-1 on Tuesday.