Terms of the deal were not disclosed.
Portland waived Miles in April after a doctor appointed by the
league and the NBA's players' association determined the damage to his
knee was severe enough to qualify as a career-ending injury.
Celtics general manager Danny Ainge said in a statement Friday
that the team worked out Miles twice and was impressed with his
health and attitude.
"Darius ... has impressed us with his progress, health, and attitude," Ainge said. "Darius will have the next couple of months to prove to myself and Coach [Doc] Rivers that he can help us win."
Miles, who was drafted out of East St. Louis High School, has posted career averages of 10.6 points, 5.2 rebounds and 1.15 blocks in 412 NBA games.
"I've watched the Boston Celtics play a lot last year and loved what I saw in their teamwork and chemistry on the court, and it's the team that I want to try to resume my career with," Miles said in the team's statement. "I am excited about having an opportunity to play on a team that I feel my personality fits with and a team that can have great success on the court."
Also Friday, the Celtics signed second-round draft pick Bill Walker. Financial terms of the deal were not disclosed.
Information from The Associated Press was used in this report.