Linebackers coach Mark Duffner will serve as the interim defensive coordinator. Duffner was the Cincinnati Bengals' defensive coordinator in 2001 and 2002.
"I have the utmost respect for Mike Smith as a man and as a football coach," Bucs coach Dirk Koetter said in a statement. "These decisions are always difficult, but our top priority here is to ensure that we do everything possible to help this team succeed.
"As I have said in the past, the issues we have had as a team are never one person's fault. During good times, as well as the bad, it is a collective effort between the coaches and the players. We all understand that this is a result-based profession and our results to this point have not met our standards. I want to thank Mike for all the hard work and passion he has displayed here on a daily basis and I wish him well moving forward."
The Bucs defense surrendered 416 yards of offense against the Falcons and allowed them to go 3-for-3 in the red zone Sunday. Tampa Bay fell to 2-3 after an electrifying 2-0 start; things cooled when the Bucs' high-powered offense slowed.
The Bears game was so bad that an online petition calling for Smith's firing was set up by fans. Koetter and the Bucs were widely criticized when the team did not make a change during the bye week. Media members questioned whether Koetter would fire Smith considering their history, having worked together with both the Jacksonville Jaguars in 2007 and the Falcons from 2012 to 2014.
Koetter said at his news conference Monday that he did not make a change during the bye week because he wanted to give the team a chance to recover from injuries and he wanted to see what adjustments could be made.
Koetter, who said the decision was solely his, called dismissing Smith the "second-most difficult" thing he has dealt with during his career, behind the 1999 death of player Paul Reyna while Koetter was coach of Boise State.
"It's difficult when you let (even) a practice squad player go," Koetter said. "Mike and I worked together in Jacksonville as coordinators and I worked for Mike in Atlanta. So I never did see this day coming, but it's here so we have to make the best decisions for our football team moving forward. That's all you can do."
The Bucs were also criticized in the offseason for not making a change at defensive coordinator after their defense gave up 378.1 yards per game last year -- last in the league. The one change they made was firing defensive line coach Jay Hayes after they finished with just 22 sacks.
The last time the Bucs made an in-season move at defensive coordinator was relieving Jim Bates of his duties in 2009 under Raheem Morris. The Bucs stripped him of his title but kept him on their staff as a consultant. Lovie Smith also demoted Leslie Frazier from the majority of play-calling duties, which Smith took over, in 2015 but he still kept the defensive coordinator title.
"It's a tough business. It's hard to see a guy like that go because I know what type of person he is outside of football," linebacker Lavonte David said. "He's a stand-up guy, never threw [anybody] under the bus, he was always a positive guy. It's sad to see a person like that go, especially one who has great character."
The Bucs allocated $88 million in 2018 cap space to their defense, sixth-most in the NFL, according to ESPN's Roster Management System.
Through five games this season, Smith's defense was giving up 439.8 yards per game. By comparison, former head coach Lovie Smith was fired after his defense gave up 354.7 yards per game over two years.
This season, the Bucs rank last in a number of defensive statistical categories: points per game (34.6), passing yards per game (356), red zone touchdown percentage (94 percent) and opponent quarterback rating (80) -- the highest since QBR was first tracked in 2006.
Sunday was the 20th time Smith's defense surrendered over 400 yards of offense during his 37-game tenure. The Bucs have also surrendered 306 explosive plays (rushing plays for 12 or more yards and pass plays of 16 or more yards), third-most in the league during that span.
According to ESPN Stats & Information, the Bucs have generated the lowest pressure rate in the NFL since Smith was named defensive coordinator, generating pressure on just 23.1 percent of dropbacks.
Koetter cited Duffner's experience -- he's the most-experienced assistant on Koetter's staff, with 22 years in the NFL -- as the key reason for the promotion. Duffner will continue to coach the linebackers.
"A lot of energy, a lot of excitement -- that's what we get from him," David said of Duffner. "I expect a lot of guys to play their best for Coach Duff. Everybody -- offense and defense -- loves Coach Duff's energy, his positivity. You can expect a lot of fun football going on."