Ruben Castro's goal midway through the second half decided things in Betis' favour as they kept the heat up on stuttering Malaga in sixth spot.
For second-bottom Celta though, Sunday's result was another damaging setback following their recent mini-revival and they are now three points adrift of safety with just two games to go.
Celta had given themselves hope of avoiding an immediate return to the Segunda Division after picking up seven points from three matches but that run came to an end with last weekend's home defeat to Atletico Madrid and there was more disappointment on Sunday.
Both sides had chances to take a first-half lead but Castro was denied by a finger-tip save from visiting keeper Javi Varas in the 21st minute while Celta forward Michael Krohn-Delhi's 31st-minute effort was parried by Betis custodian Adrian.
The match followed a similar pattern at the start of the second half, with Castro and Krohn-Delhi again drawing saves from their opposing goalkeepers.
It was Castro who finally made one of his chances count though as he put Betis ahead in the 66th minute with a right-footed finish into the bottom corner following a cross from Francisco Chica.
Castro almost made it 2-0 five minutes later but this time he was thwarted in his latest duel with Varas.
Adrian was also earning his wages as well, and he produced a vital save to turn Borja Oubina's 75th-minute strike round the post.
That would be the closest Celta would come to an equaliser as substitute Danijel Pranjic failed to hit the target with his late opportunity.