Saturn replaces its entry-level model line for 2003 with larger, more-powerful sedans and a 4-dr coupe. Ion is Saturn's first fully redesigned car since it opened for business in 1990. It replaces the S-Series and retains the brand's dent-resistant plastic body-side panels. The Ion sedan and coupe are nearly the same length, and both are significantly larger on the outside than their S-Series counterparts. The sole engine is a new 4 cyl with 140 hp, 16 more than the top S-Series engine. A 5-speed manual transmission is standard. Two automatic transmissions are available. Sedans get a conventional 5 speed. Coupes use a new continuously variable unit (CVT) with near-infinite gear ratios. Head-protecting curtain side airbags are optional; torso side airbags are unavailable. All Ions have front-disc/rear-drum brakes. An ABS/traction-control system is optional.

Ion's Quad Coupe has two rear-opening half doors behind the front doors. They do not open or close independently of the fronts. The S-Series coupe had one such rear door. As a design touch, Saturn dealers offer interchangeable strips of contrasting-color body trim tracing the arc of Ion's roof. They can be matched to interchangeable interior trim. Ion locates its main instrumentation in the center of the dashboard rather than in front of the driver. Split folding rear seatbacks are standard. The coupe's front passenger seat folds forward to create a continuous 10-foot-long storage channel. Ion comes in trim levels labeled 1, 2, and 3. Air conditioning is standard on all except the 1 sedan. Leather upholstery is optional on 3 sedans and 2 coupes. Wheel sizes are 14, 15, or 16 inches, depending on model.