Romanowski has missed the last four games after a series of concussions earlier in the season that ended his streak of appearances at 243 consecutive games. The 37-year-old, who has played in five Super Bowls, never missed a game before this season.
"It's a hard decision. The facts dictated the decision," coach Bill Callahan said. "We haven't discussed any additional seasons or retirement at this point."
Romanowski left the Raiders' training complex before Callahan's announcement. He will remain with Oakland for the rest of the season as an unofficial defensive adviser, helping the team and his replacement, Travian Smith.
"I don't think there's been a more dominant player than Bill at that position," Callahan said. "He knows all the intricacies of the position, how to play that role. He understands blocking schemes as well as anyone I've ever been around. We'll miss the chemistry he brings. We'll miss his leadership, as well."
Romanowski, in his second season with the Raiders, also has played for Philadelphia, San Francisco and Denver. His high-profile career has included plenty of intimidating hits and a fantastic regimen of physical preparation -- as well as a history of questionable behavior, including an infamous fight during training camp two months ago.
Last week, Romanowski was sued by teammate Marcus Williams for punching the second-year tight end in the face during the fight. Williams also is on injured reserve, with a broken orbital bone.
The suit, filed in Alameda County court, seeks unspecified damages for alleged battery, negligence and intentional infliction of emotional distress. Romanowski has apologized for his actions.
Romanowski has undergone dozens of tests with several teams of doctors in recent weeks to examine his health, and doctors apparently advised him against returning to the field this season. He is expected to visit specialists for several more months before making a decision on his future.
In addition, if Romanowski retires before the end of the season, it would cause serious salary cap problems for the Raiders by accelerating the application of his signing bonus to Oakland's salary limits.
"I was still optimistic that the results would show he could possibly play," Callahan said. "But unfortunately, not."
The Raiders will consider several options to fill Romanowski's roster spot. Players at several positions will audition this weekend for the team, which is on its bye week.