Each NFL team has the opportunity under the collective bargaining agreement to lock up the services of one player on its roster by paying that player an average of the league's top five salaries at that position.
The franchise tag ensures Welker will stay with the Patriots for one season, although the Patriots said in a statement the team will pursue a long-term contract with Welker.
"Wes Welker is a remarkable football player for our team and has been a vital component to our offense and special teams since we traded for him in 2007," the statement said. "Utilizing the franchise designation allows both sides more time to try to reach an agreement, which is the goal.
"Wes remains a contractual priority and we are hopeful that he will remain a Patriot for years to come."
Welker was third in the NFL in receptions last season and set a franchise record with 1,569 receiving yards. He also had a career-best nine touchdowns.
Other players declared franchise players Monday included linebacker Anthony Spencer by Dallas, defensive end Cliff Avril by Detroit, kicker Josh Scobee by Jacksonville, kicker Phil Dawson by Cleveland and tight end Fred Davis by Washington.