Grimes, who led the team with 17 passes defensed in 2011, is one of the most important names in an unusually large class of 17 unrestricted free agents. He had a combined 11 interceptions in 2009 and 2010 and was an alternate Pro Bowl pick in 2010.
Grimes' agent, Ben Dogra, said the decision by Falcons general manager Thomas Dimitroff was expected.
"We anticipated receiving the franchise tag," Dogra told The Associated Press. "Brent Grimes is certainly viewed as one of the elite cornerbacks in the NFL, and getting franchised is not a surprise to any of us. We look forward to continuing to speak with Thomas Dimitroff over the next few days."
Dimitroff announced the move on the team's radio partner, Atlanta's WQXI AM-790. He said he plans to continue talks on a long-term deal with Grimes.
"Our feeling is, Brent Grimes is an excellent athlete with the ability to move, cover, break on the ball with some of the top corners in this league," Dimitroff said. "We feel comfortable with his abilities, the way that he's picked up our system and settled in nicely in the few years since we have been here."
"We feel like, as we all know, you're always looking for corners that can settle into your system and cover and stay in phase, as what we call it often, and make big plays on the ball. I think Brent Grimes does that, obviously, against big, small, fast or slow receivers in this league. We're encouraged about having him back."
Dogra said Grimes hopes "to explore all options with the Falcons, including signing a long-term deal."
Dimitroff was scouting players in California on Friday and could not be reached for additional comment.
Grimes (5-10, 183) was an undrafted free agent from Shippensburg who emerged after joining the team's practice squad in 2007.
Other top Atlanta free agents include linebacker Curtis Lofton, who led the team in tackles, safety Thomas DeCoud, who topped the Falcons with four interceptions, defensive end John Abraham, who led with 9 1/2 sacks, and center Todd McClure. Other free agents include backup running back Jason Snelling, backup quarterback Chris Redman and receivers Harry Douglas and Eric Weems.
Dimitroff said after the season it might be difficult for the team to keep more than eight free agents.