A fourth-minute penalty from Piti got the hosts off to a flying start and they might have been even further ahead by the break had Roberto Trashorras not seen a fine effort come back off a post.
Andrija Delibasic would eventually double Rayo's advantage, heading home Jose Casado's cross at the end of a wonderful team move midway through the second-half, and Jose Carlos rounded off the scoring with a neat finish eight minutes from time.
Betis had been awarded a penalty inside two minutes of a 1-1 draw with Athletic Bilbao but they suffered the same fate early on when Adrian raced off his line to bring down Delibasic as the Montenegrin tried to round him.
Piti stepped up and, on his 200th appearance for Rayo, he just managed to squeeze his effort underneath the unfortunate goalkeeper.
Rayo continued to boss the early stages and could have been 2-0 up when a lovely dipping effort from Trashorras came back off the inside of the post.
Betis grew into the game as the half wore on, though, and after Joel Campbell had fired a warning shot straight at Ruben in the Rayo goal, Ruben Castro should have equalised but he dragged his shot wide with just the keeper to beat.
Adrian then had to be alert to prevent a Piti corner bending in at the back post and Lass Bangoura also missed a glorious opportunity to double the hosts advantage five minutes into the second-half, firing wide at the end of a flowing counter-attack.
The second goal would arrive with 26 minutes remaining, though, and when it did it was a thing of beauty. Alejandro Dominguez's backheel released Casado down the left and his cross was rocketed home by Delibasic.
Ruben then produced fine saves from Salva Sevilla's free-kick and from Castro to prevent Betis getting a late foothold in the game, with Carlos sealing a fine afternoon for Paco Jemez's men by slipping past two challenges inside the box to curl the ball into the far corner.