“Paul holds immense county and international experience,” Foxes director of cricket Claude Henderson said.
“He offers the ability to score quick runs at the top of the order, as well as take wickets.”
Leicestershire, seeking a fourth T20 title and first in 13 years, are third in their group going into this weekend’s two East Midlands derby games against Notts and then Derbyshire on Sunday.
“We’ve got two huge games coming up,” Stirling added.
“And, with the way the table’s shaping up we’re in a great position. Hopefully we can get a win or two more and put ourselves in strong contention for a home quarter-final.”
Stirling has played 355 T20 games in his career – most recently for Ireland at the World Cup in the Caribbean – and scored almost 8,500 runs.
He has twice won the Men’s Hundred, with Southern Brave in 2021 and again last summer for Oval Invincibles.
He made 313 runs in 13 games for the Bears in 2022, including a stunning 46-ball century on his debut, before returning to make two more appearances for the Edgbaston-based side in 2023 – the last of them in a win at Leicester.