The firings of Smith, secondary coach Jon Hoke and defensive line coach Jethro Franklin came after Houston finished 8-8 for the second straight year.
Houston's offense finished the year ranked third in the NFL, but the defense was 22nd, allowing more than 336 yards per game. Only five teams allowed more points per game than the Texans' 24.6. Houston's defense improved late in the season but didn't live up to expectations for a unit featuring 2006 top pick Mario Williams.
The other first-rounders on Houston's defense are lineman Amobi Okoye and Travis Johnson.
Smith came to Houston in 2006 after working one year as Miami's defensive coordinator. Before that, he served two seasons as the assistant head coach and linebackers coach for Detroit.
Hoke had been the Texans' secondary coach since the team's inception in 2002. Houston's pass defense ranked 17th and generated only 12 interceptions in 2008.
Franklin finished his second season with Houston. The Texans' 25 sacks ranked 27th in the league, though Williams was named to his first Pro Bowl.
"Obviously, it didn't meet expectations. In this business though, you can't be too surprised when something like this happens," Franklin told the Houston Chronicle.
Houston coach Gary Kubiak told reporters Monday that he planned to meet with team owner Bob McNair and begin evaluating all coaches.
"It's part of our job, part of our business," he said.
"There's tough decisions to make, and like I said, I've got to get through this period and do a good job on my evaluations and do what's best for our football team as we move forward."
Kubiak said Monday he expected assistant defensive backs coach Ray Rhodes to return next season. He also hoped to retain assistant head coach Alex Gibbs, who mostly worked with offensive coordinator Kyle Shanahan.
The Texans won five of their last six games, allowing only 18 points per game in that span. They held Tennessee to 281 total yards in a 13-12 win Dec. 14 and allowed only 294 yards against Chicago in the season finale, knocking the Bears out of playoff contention.
Houston's defense ranked 24th in 2007, but this year's sack total was down from 31 in 2007. The Texans ranked 23rd against the run (122.6 yards per game) after ranking 19th (114.1 yards) in the category last season. Kubiak said Monday that he had to look at the defense across the entire season, not just during the late surge.