With 33 points from 36 matches, Leganes are five points above the drop zone. Betis, already safe from relegation, remain in the 15th spot with 37 points.
The hosts jumped out to a 1-0 lead in the 6th minute when Alexander Szymanowski converted from the spot after Betis' Alex Martinez was cited for a hand ball.
Nabil El Zhar put Leganes up 2-0 in the 15th minute, scoring courtesy of what looked like a harmless deep ball from teammate Diego Rico.
The third goal for the home side arrived in the 64th minute on a magnificent strike by Gabriel from more than 40 yards.
The closest Betis came to scoring was an Alex Alegria header saved by Iago Herrerin minutes before Szymanowski scored his second of the match to complete the 4-0 drubbing. EFE