Fire crews were alerted to the blaze shortly before 1pm today.
A total of three engines from Walsall and Tettenhall were sent to tackle the flames at the Asda store off Jack Hayward Way, across from Molineux.
The fire, which was confined to the private hire taxi's engine compartment, was easily put out and firefighters had left by 1.17pm.
Nobody was injured in the incident.
A spokesperson said: "Fire crews attended to find one private hire car on fire on the car park of the Asda Superstore.
"The fire was in the engine compartment only and had been caused by a vehicle defect.
"The vehicle's owners was arranging for collection of the vehicle."