The team announced Harvin formally signed his contract Wednesday. It's a one-year deal initially agreed to last week.
Harvin is a dynamic receiver and special-teams returner, who joins his fourth team in two years. He opened last season with Seattle before being traded to the New York Jets.
As for Clay, the Bills announced they have made an offer to sign the fourth-year player.
A person familiar with negotiations told The Associated Press on Tuesday that Clay signed the Bills' five-year, $38 million contract offer. The person spoke on the condition of anonymity because the Bills have not revealed the terms.
The Miami Dolphins have five days to match Buffalo, or lose Clay without compensation.