The Scottish Premiership club reported on their website that the 23-year-old Englishman will join on loan until the end of the season.
Thistle boss Alan Archibald said: "I'm delighted to have managed to bring Lyle to Firhill where he will complement the strikers that we already have at the club.
"He did really well with Falkirk scoring a barrow load of goals and that earned him his move to Sheffield United.
"Things haven't worked out for him with United, so far, which can happen when there is a change of manager but he carries a potent threat and has a real eye for a goal.
"He's quick and he is good getting in behind defenders as well. I'm sure he will prove to be an important addition to the squad for the second half of the season."
He is expected to start training on December 31st and will be part of the Thistle squad to face Celtic on New Year's Day.
Lyle is a former Millwall and Bournemouth player and, while with Falkirk, was voted the SPFA's First Division Player of the Year.