"Elfsborg are changing head coach as a part of the build-up for 2014," the club said on its website on Monday.
Former Sweden international Klas Ingesson, a youth coach at the club, will take over the reins for the rest of the year.
"Jorgen has done a fantastic job as head coach," club chairman Bosse Johansson said. "I want to emphasise that this is a pro-active change ahead of the new, big challenges in 2014."
Having won the Swedish top flight title in 2012, Lennartsson's side failed to reproduce the form this season and are currently sixth in the table with four games left to play, 12 points behind leaders Malmo FF.
A 1-0 aggregate defeat by Celtic meant the Boras club failed to qualify for the Champions League group stage.