Naseem will act as cover for Afghanistan overseas Naveen-ul-Haq, who is currently involved in the IPL play-offs with Lucknow Super Giants. Naseem was due to arrive in England on Tuesday and be available for Leicestershire's Blast opener against Lancashire on Thursday.
"Having Naveen in the IPL play-offs meant we had to make a quick call and we are extremely excited to have secured someone of Naseem's calibre," Leicestershire Director of Cricket, Claude Henderson, said.
"He's proven to be a world-class performer in T20 cricket, and we look forward to seeing him in a Leicestershire shirt shortly."
Leicestershire start their campaign with back-to-back games, away to Lancashire and then at home to Warwickshire, followed by a trip to Worcestershire on Monday. Should Lucknow reach Sunday's IPL final, then it is likely Naveen would miss all three fixtures.
Naseem featured for Gloucestershire in the Blast last season, taking five wickets in three games. He was also signed by Welsh Fire to play in the Hundred, but withdrew due to international commitments.
Naseem said: "I'm very excited to be joining the Foxes for the start of the Vitality Blast. I can't wait to meet my team-mates and help win games for Leicestershire."