Brennan, who was contracted to French club Toulouse, has made the decision just days before he is due to face an independent disciplinary committee relating to allegations that he hit a supporter when Toulouse played Ulster.
Brennan is alleged to have entered a seating area during Toulouse's 28-13 European Cup victory over Ulster last month and punched a supporter.
There is a chance his professional career would have been brought to an end with a lengthy ban at the hearing, on Friday, but the lock has pre-empted any such decision.
Brennan had tried to have his ERC hearing adjourned pending a separate civil case but that appeal was turned down, forcing him to make the announcement.
He said: "I had decided at the beginning of this season that I would retire at the end of the current French Championship.
"However, as a result of recent matters I've had to review that position. I had sought an adjournment of the ERC disciplinary hearing on the grounds that it interfered with my right to silence in other proceedings, but this has been refused.
"I am therefore announcing my immediate retirement from rugby. I would like to thank everybody who has helped me throughout these years," he added.