The Seville-based club moved on to 76 points from 28 matches with a 3-1 victory over Tenerife on Wednesday, and Celta were the only club who could mathematically deny them promotion.
With four matches left to play, Betis are now 13 ahead of third-placed Granada. The top two clubs go up automatically.
Betis won La Liga back in 1935 and the King's Cup in 1977 and 2005, and competed in Champions League in the 2005/06 campaign before being relegated in 2009.
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