The Munster number nine has won 98 caps, played in three Heineken Cup finals and made more than 200 appearances for Munster.
Bath head coach Gary Gold said: "Peter is a world-class professional and has an incredible amount of experience he can bring to the squad."
He added: "He is still keen to compete at the highest level, and we are short at scrum-half at the moment due to injury.
"I am sure he will be a huge asset to our other nines with his invaluable experience and exceptional work ethic."
- Sports & Recreation