The 26-year-old will be available for Lancashire’s final two round matches and any knockout games should the side qualify.
He last featured in county cricket competition with Yorkshire in 2016, scoring 384 first-class runs at an average of 54.85.
Fellow Redback Joe Mennie played seven List A and seven first-class matches for Lancashire last summer.
“I am hugely excited to have been given the opportunity to join a massive club like Lancashire,” Lehmann said on the club’s website.
“I thoroughly enjoyed my previous spell in England with Yorkshire a couple of years ago and I’m really looking forward to pulling on the Red Rose and playing at Emirates Old Trafford this time around.
“I am looking forward to meeting all of the lads and I hope that I can put in some match winning performances to help the team finish strongly in the Royal London One-Day Cup.”
Lehmann replaces fellow Australian Glenn Maxwell in the Lancashire squad with the Victorian summoned home to join Australia’s World Cup squad ahead of next month’s tournament.
Several other Redbacks, including Callum Ferguson and Daniel Worrall, have signed with county cricket teams for the English summer.
Others such as Jake Weatherald (Cuckfield) and Nick Winter (Brentwood) will also continue to hone their craft at club level in the UK.