The 28-year-old has been a key man for Mark McGhee's side this season and will now extend his second spell at Motherwell until the end of the 2009/10 campaign.
"I am just delighted to have got things sorted out," Lasley said.
"Motherwell have been so important in my career and I love the club and the place.
"There were one or two wee things to sort out before I could get it signed off, but I am glad to say that it's been ironed out. This season has been brilliant for me - to play such a big role in a team doing so well.
"We've got a real chance of achieving something big this term and I want to be here to not only achieve that, but to reap the rewards afterwards."