Harvin retired in April after spending last season in Buffalo, citing numerous injuries. He had 19 catches for 218 yards and a touchdown in five games with the Bills before being sidelined by lingering hip and knee injuries.
Harvin fills an immediate need on a team with a banged-up group of receivers. Buffalo played New England on Sunday without three of its top four receivers, then lost backup Brandon Tate to a possible concussion. Sammy Watkins is on injured reserve with a left foot injury, but is eligible to return later this month.
Harvin, 28, must still pass a physical. When healthy, he was a dynamic playmaker.
Harvin retired with 351 receptions, 4,936 yards of offense and 32 total touchdowns. His best season was in 2009 with the Vikings, when he caught 87 passes for 967 yards. But his production dwindled after that.
"That's my boy, I'm definitely excited," quarterback Tyrod Taylor said. Taylor and Harvin grew up in the Virginia Beach, Virginia, area and played high school football and basketball against each other.
Before he retired, the Bills were Harvin's third team in two years after he was traded by the Seattle Seahawks to the New York Jets during the 2014 season.
The Minnesota Vikings drafted Harvin in the first round in 2009 after he helped Florida win two national titles in three seasons. He was the AP offensive rookie of the year in 2009, finishing his first season with 60 receptions and eight touchdowns, two on kick returns.