The Dutchman, capped 109 times by his country, left Hamburg after a disappointing season in the Bundesliga and will now try his luck in La Liga for the second time.
After his second spell at HSV ended, the 32-year-old rejected the chance to rejoin his first team Ajax in order to sign for newly-promoted Betis, admitting that his Spanish roots had an influence on his decision.
"I'm very happy to come here. It's a great club and I really can't wait to start," the former Real Madrid player said. "For me it is a dream because my family lives in Chiclana, which is very close, and I like playing in Spanish football.
"I always give 100 per cent when I play. I want to win and score goals, besides giving assists. This stadium is always full and that's why I really want to win many games.
"I have a lot of experience. I have to play well. We have to enjoy it because football is the best job in the world."
Betis won promotion this season and Van der Vaart is their first signing ahead of next term's top-flight return.