The 32-year-old, who captained the team in the latter stages of the season, follows fellow long-serving player Stevie Hammell in staying at Fir Park with both players awarded a testimonial match.
Lasley has played 298 games for the club in two spells separated by a stint at Plymouth and has helped the club into the Champions League qualifiers this year.
Leeann Dempster, the Motherwell chief executive, said: "We are thrilled Keith has joined Stevie Hammell in staying on at the club.
"Keeping our experienced players, who are among our most consistent performers, was obviously high on our priority list. We are still negotiating with others but we hope both players staying will send out a message of intent."
Lasley said: "I am delighted – I never wanted to leave Motherwell.
"It was similar to any contract, there were negotiations back and forth but I am just so pleased it has been sorted out. I am already looking forward to next season.
"There was other interest and a few other interested clubs and obviously you have got to think about your family," he added.