The 27-year-old, who passed a medical at the club on Monday, has revealed that he is "very happy" to sign a three-year deal with the Spanish outfit.
"I am very happy to play for Betis. This is a very important step in my life and in my football career," he told the club's official website.
"I know I am at a big club and a in a spectacular League. Betis have been a club that have been following me for a long time and I have to thank them.
"They were seventh in a league as demanding as La Liga last season and this year they will be in Europe. These are some of the reasons as to why I have decided to come here."
Steinhofer has previously represented Bayern Munich.