The Monte Carlo outfit had temporarily slipped to second following Paris Saint-Germain's 1-0 win at Valenciennes earlier in the evening, but goals from Emmanuel Riviere and a Radamel Falcao brace helped Monaco leapfrog the champions.
Despite a slow start to the match, the hosts had the first real chance when Andrea Raggi crossed to Riviere, who hit a powerful first-time volley at goal, but was stopped by a save from Mickael Landreau.
Lucas Ocampos came close to a spectacular opener just three minutes later, but his overhead kick from a corner was tipped over by Landreau.
However, Monaco broke the deadlock with two goals in quick succession just before the break.
The home side went ahead through Riviere, after he picked up a pass inside the box from Falcao before slicing an effort into the bottom corner of the net.
Monaco quickly doubled their lead just two minutes later when Falcao collected the ball in midfield and easily passed the Bastia defenders before smashing his finish into the back of the net.
Falcao nearly scored his second after half time, but his thunderous shot was saved once again by Landreau.
Bastia nearly pulled one back in the 57th minute when Boudebouz lashed an effort towards goal, but Subasic pulled off a fine stop to deny the midfielder.
Subasic was called into action again five minutes later when he expertly saved a Wahbi Khazri rocket from 25 yards out.
Falcao made sure of the win in the dying minutes with a smashing strike from outside the box.
Monaco climb back to the top of Ligue 1 with the win, while Bastia drop to 11th.