Mason, who met with coach John Harbaugh and general manager Ozzie Newsome on Saturday to discuss the matter, was expected to send a letter to the NFL stating his intentions to rejoin the team.
"It was a tough decision, but I think it was a good decision for me to come back," Mason said in a statement. "I needed to evaluate my life, football and my career.
"I still have an intense fire inside me, and I want to play. … I felt like I had left something undone, and I wanted to finish it. I believe this is a good example for my son and my daughter on how to be thoughtful and also follow through."
He called Ravens owner Steve Bisciotti on Saturday morning to express his decision to return.
He was taking a physical Saturday afternoon and was set for a conditioning test afterward, with intention to suiting up for practice Sunday.
Mason retired less than a week after the murder of close friend Steve McNair.
The loss of his friend, family pressure, and the team's unwillingness to adjust his $3 million salary combined to lead to a surprise retirement announcement July 13.
Harbaugh talked to Mason after the announcement and said he was optimistic that Mason would return. Camp started without Mason, but Harbaugh left the door open for Mason's arrival.
The 35-year-old played a key role in helping Baltimore reach the AFC Championship Game last season. He started all 16 games and led the Ravens in receptions (80) and yards receiving (1,037). He then added 12 catches for 190 yards and a touchdown in three playoff games.
Before Mason's retirement, the Ravens had been in the market for another veteran receiver. His retirement had put them in need of two receivers.
John Clayton is a senior NFL writer for ESPN.com. Information from The Associated Press was used in this report.