The 61-year-old Rust, Pittsburgh's defensive coordinator last season, had been the Patriots' defensive coordinator from 1983 to '87, the last 3 1/2 years under Berry. Patriots General Manager Pat Sullivan fired Berry on Monday after a disagreement over the hiring of an offensive coordinator.
Minnesota Vikings' Coach Jerry Burns, at 63, is the NFL's oldest head coach.
Rust returned today to a team that finished 5-11 and missed the playoffs for a third straight season. The coaching change came a month after Sullivan had given Berry a vote of confidence.
But events escalated swiftly last week when Sullivan pushed Berry to make changes. Berry wanted to let current assistants handle the offensive coordinator's job, and Sullivan got permission last Friday from owner Victor Kiam to fire Berry.
With the draft less than two months away and a possible March 1 deadline for hiring another team's assistant, the Patriots made a move. An NFL guideline states that no team can seek permission after March 1 to hire another team's assistant to become its own assistant. A league spokesman said that probably could apply if the new job is head coach.
So Sullivan turned to a familiar face. Rust, an assistant with New England who went to its only Super Bowl in 1985, was named head coach on Berry's 57th birthday.
Rust became defensive coordinator in 1988 under Kansas City Coach Frank Gansz but lost the job when Gansz was fired, and moved to Pittsburgh in 1989.
In taking his fourth job in four years, Rust rejoins a club he had left temporarily in one of its more bizarre situations.
Midway through the 1984 season, on Oct. 24, Patriots Coach Ron Meyer fired Rust before telling Sullivan, who was in New Orleans at league meetings. Sullivan rushed home, hired Berry that night and fired Meyer the next morning. One of Berry's first acts as head coach was to bring Rust back as defensive coordinator.
Like Berry, the soft-spoken Rust is not outwardly emotional or revealing and rarely criticizes his players.
But while Berry's unimaginative offense sparked criticism of his football knowledge, Rust's constant employment in key coaching positions leaves little doubt about his ability.
"He gets a lot of respect because of his intelligence," Pittsburgh linebacker Coach David Brazil said.