Overkill's Ulf Andersson described it as an in-depth collaboration between the two gaming franchises, without revealing specific details.
"As perceptive gamers will have noticed, several hints have recently been dropped into Payday: The Heist, which has led to various rumours. We are excited to be able to confirm that an in-depth collaboration between Overkill and Valve is currently in production," revealed Andersson.
"We are working on a very cool blend of Payday and Left 4 Dead. I am sure it is so exciting that it will have some players check into the hospital before we are done," he concluded.
Payday: The Heist is a first-person shooter that lets players take part in bank robberies, prison breaks and drug ring infiltration in teams of four.
It can be downloaded for the PlayStation 3 and PC.
Making its debut in 2008, Left 4 Dead sees teams of four strangers attempt to survive a zombie outbreak.
A sequel was released in 2009, followed by a number of additional levels and gameplay enhancements.
> Read Digital Spy's hands-on preview of Payday: The Heist
> Read Digital Spy's Left 4 Dead 2 review
Watch a trailer for Payday: The Heist below: