The 26-year-old joins for an undisclosed fee from Wolfsberger where had previously worked with Reds head coach Gerhard Struber and has signed a contract until 2022.
He will be the third oldest player at Oakwell behind fellow Austrian Sami Radlinger and out-of-favour left-back Dani Pinillos as Struber looks to add experience to the squad which is embroiled in a relegation battle.
Ritzmaier was signed by Dutch giants PSV Eindhoven as a teenager and spent eight years there, playing 15 matches and being loaned out to other Eredivisie clubs Cambuur, NEC Nijmegen and Go Ahead Eagles.
Ritzmaier – who can play anywhere in the midfield diamond and left-back – returned to his home country in 2018 to play for Wolfsberger for whom he netted five goals in 57 games. He has played in the Europa League for both PSV and Wolfsberger.