Allmendinger had been suspended since July 7 after flunking a NASCAR administered drug test at Kentucky Speedway 10 days earlier.
The driver's representatives said he had tested positive for amphetamines and had chosen to participate in NASCAR's Road to Recovery program.
Allmendinger forfeited his spot as driver for the No. 22 Penske Racing Dodge after the suspension. Roger Penske told NASCAR.com last weekend he would consider rehiring Allmendinger for one of his IndyCar seats.
"I want to thank everyone for their support through this entire process," Allmendinger said in a statement. "I appreciate that NASCAR created the Road to Recovery program, and am grateful for the opportunity to return to competition."