Ulbrich has been contemplating retirement since sustaining a severe concussion during an Oct. 11 game against the Atlanta Falcons. He was placed on injured reserve eight days later.
Ulbrich said Friday that the time had come to make the decision. He plans to pursue a career in coaching, perhaps even with the 49ers.
"I don't like to say it, but this is it," Ulbrich said. "I think it was a medical thing. But I'm excited about coaching and ready to start that next phase of my life."
Ulbrich, 32, played in 121 games for the 49ers with 75 starts. He finished his career with 653 tackles, including a career-high 167 in 2004. He also had 5½ sacks, five forced fumbles and two interceptions.
San Francisco's third-round draft pick in 2000, Ulbrich is one of 47 players in franchise history to play 10 years or more with the 49ers.
Ulbrich said he will seek coaching opportunities at either the professional or college level.
San Francisco defensive coordinator Greg Manusky, who played linebacker in the NFL for 12 seasons before entering coaching, said Ulbrich won't have any difficulty finding work.
"He'll make a great coach in the league," Manusky said. "He's a student of the game. He understands defenses, and his personality and his type are pretty dang good."