The 25-year-old midfielder had received approaches from Scottish Premiership rivals Dundee United and Inverness Caledonian Thistle.
Manager Alan Archibald said: "Steven has proved over the last few seasons what a valuable part of the squad he is.
"It came as no surprise that Steven had offers from other Premiership clubs."
Lawless, who joined Thistle from Motherwell in 2012, made 40 appearances this season, scoring five goals.
Thistle are also waiting to hear from Frederic Francs and Gary Miller after contract offers to the defenders.
Caley Thistle, who finished two places above the Glasgow side in seventh, had also made approaches to Lawless' midfield team-mates, Stuart Bannigan and Abdul Osman.
Bannigan rejected a new contract at Firhill along with defender Danny Seanborne, but Osman signed a new two-year deal.
Steven Lawless has extended his contract with Partick Thistle for a further two years.