The 27-year-old Englishman spent a season on loan at Fir Park from Rotherham United before signing a permanent contract last summer.
Now he has agreed a new one-year deal to stay in Ian Baraclough's squad.
"Well have managed to beat off stiff competition from SPFL Premiership and English clubs," said the club website.
Ainsworth scored in both legs of Motherwell's Premiership Play-off final victory over Rangers to take his tally to 22 in his two seasons at Fir Park.
He told the club website: "Last season was disappointing in many ways, albeit we finished strongly against Rangers in the play-offs.
"I think there is a real desire in me and the guys who are still at the club to ensure next season is much better."
Ainsworth, whose previous clubs include Derby County, Watford and Huddersfield Town, said he is relishing the use of two wingers - himself and Marvin Johnson - since the arrival of Baraclough as manager.
"Obviously the way the team plays now, with pace all across the front area, really suits me, with Marvin on one side and me on the other," he added.
Lionel Ainsworth has signed a new contract with Motherwell, who say they faced competition from clubs around Britain who also wanted the winger.