The on-loan striker nearly doubled his tally moments later but he couldn't apply the finish when Callum McGregor's deflected shot fell to his feet.
The Bhoys have been on top for most of the first period, and earlier in the half, Kris Commons went close with an effort after Guidetti had fired wired.
Anthony Stokes thought he had given Celtic a fifth-minute lead but his shot flew just wide after a turn on the edge of the box.
Morgaro Gomis nearly made it 2-0 with an own goal shortly after the half-hour mark but Hearts keeper Neil Alexander spared his teammate's blushes with a reflex save.
Kris Commons made no mistake with the spot kick before Adam Eckersley put through his own net to make the result safe on the hour mark.
Celtic thought they had made it 4-0 towards the end of the game, but Aleksandar Tonev was flagged offside after the ball had struck him on its way to goal.