The 27-year-old had fallen out of favour at the Bundesliga side following Peter Stoger's arrival as manager and subsequently found himself linked with Premier League rivals Manchester United and Manchester City, as well as a potential return to former side Barcelona.
Last night Betis confirmed that they had secured a long-term deal for the Spaniard, however, after agreeing an undisclosed fee with Dortmund.
"Since my name came up as a possible candidate to come here, I started to feel the love through social media, a lot," Bartra told the Betis website. "I even have goose bumps. I received a lot of messages to come to Betis. I knew it would not be easy. But when you really want something, you feel like it, and you feel it, it all becomes easier.
"The project here is very good, it can be seen. I'm sure it will improve. The confidence they laid on me from the very first moment has been key. Since they contacted me and told me about coming to Betis, I really liked the idea. Also, the playing style and the fans is an extra. It will make me ready to help to make everything work and make a successful season."
Betis are currently 13th in the La Liga table, 10 points clear of the relegation zone.