The 26-year-old will be able to provide Reds manager Gerhard Struber with versatility in midfield and can also operate at left-back.
He will bring plenty of experience to Oakwell, having played over 250 career games and featured for PSV Eindhoven in the Europa League.
Ritzmaier has played for Austria at youth level and has made 18 appearances for Wolfsberger AC - Struber's former club - this season, scoring twice and notching four assists.
“I am really happy to sign for Barnsley FC and to play in England," said Ritzmaier.
"I have played for eight and a half years in the Netherlands and also in Austria and also the Europa League so I hope I can bring some experience to the team.
"I have a good relationship with Gerhard and Max, I know them as trainers and they know me as a player.
"The way they want to play fits me very well, I am happy to work with them.”
On Ritzmaier's arrival Struber added: “Marcel is a special player and will offer us more options for positions in the diamond.
"He can play in what I would call the number eight, the six and the 10.
"He’s a player with experience and high technical skills and is also quick, so I am very happy he’s chosen to play for Barnsley FC.
"He will be a good player for this club.”
Barnsley CEO, Dane Murphy, is pleased that the club have been able to get a new player in at Oakwell so early in the transfer window.
He said: "Many understand that this window can be difficult to get deals done, so we’re pleased to sign Marcel early.
"Marcel is an immensely-talented player with background in the highest levels of European football.
"Barnsley Football Club is lucky to have secured his services as a few clubs in the top divisions of the continent chased his signature.
"Gerhard is quite familiar with Marcel and will utilize his ability to get maximum return.
"This is another step in the right direction for Barnsley Football Club in what will be an exciting second half of the season.”