The Pontiac Custom S was a single model year car that was marketed between the Pontiac LeMans and the Pontiac Tempest. For reasons unknown, this one-year only nameplate replaced the previous model years Tempest Custom name in the Pontiac line-up.

The Custom S was only available for the 1969 model year. The car was originally to be called the "Pontiac TC".

It was available in six different body designs: two-door convertible (VIN #'s starting with 23567xxxxx), hardtop coupe (VIN 23537xxxxx), sports coupe (VIN 23527xxxxx), a 4-door sedan (VIN 23569xxxxx), a 4-door hardtop (VIN 23539xxxxx), and a station wagon (VIN #'s 23535 and 23536 that depended on how the tailgate was hinged). The sports coupe can be differentiated from the hardtop by the presence of a vent window in the front door. Interior and exterior appointments were more lavish than the base Tempest but not quite as luxurious as the LeMans and GTO.

The Custom S came standard with Pontiac's unique overhead camshaft OHC-6 175 hp 250 in³ engine. Also available was a 230 hp (170 kW) 265 ft·lbf (359 N·m) TQ OHC inline 6-cylinder with the "Sprint" package, a 265 hp (198 kW) overhead valve pushrod V8 350 in³, and a 330 hp (250 kW) version of the same engine with higher compression and a 4-barrel carburetor.

Originally planned for 1969 was a lower-priced junior musclecar to be based on the Custom S series, and a competitor to the inexpensive and fast-selling Plymouth Road Runner, which started the econo-musclecar trend in 1968 due to a starting price of under $3,000 (by this time the GTO and similar upscale musclecars had starting prices closer to $3,500 and optioned-out cars often sold in the $4,500-$5,000 range - too expensive for younger buyers for which marketing of musclecars was aimed at). This car was to be built on the two-door pillared coupe bodystyle and be powered by the 330-horsepower 350 HO V8 with other equipment including the Custom S's bench-seat interior and exterior with Carousel Red paint, Rally II wheels sans trim rings and sporty striping. This car, which was said to have been named the "Pontiac ET" (for elapsed time), was presented to division officials but rejected as presented. Instead of the lower-priced musclecar concept, Pontiac decided to retain the flamboyant paint job and striping ... and offer it as "The Judge" option on the GTO, which further jacked up the GTO's price.

The Custom S nameplate was dropped after the 1969 model year. For 1970, that series was replaced by a new base LeMans line with the same bodystyles, while the previous up-level LeMans became the LeMans Sport. The entry level Tempest continued for one more year with a new T-37 hardtop coupe added at mid-year, that included a GT-37 option package as lower-priced junior musclecar available with 350 and 400 V8s somewhat similar to the abortive '69 ET series. The T-37 nameplate replaced Tempest entirely for the entry-level Pontiac intermediate series in 1971.