| Competition: | Nationalliga A |
| Season: | 2002–03 |
| Winners: | Grasshopper Club 27th title |
| Relegated: | Luzern Delémont |
| Continentalcup1: | Champions League |
| Continentalcup1 Qualifiers: | Grasshopper Club |
| Continentalcup2: | UEFA Cup |
| Continentalcup2 Qualifiers: | Basel Neuchâtel Xamax Young Boys |
| League Topscorer: | Richard Núñez (27 goals) |
| Prevseason: | 2001–02 |
| Nextseason: | 2003–04 |
The 2002–03 season of the Swiss Nationalliga contained two divisions, each with twelve clubs. The top tier was called Nationalliga A (NLA) (French: Ligue Nationale A, Italian: Lega Nazionale A) and the second tier was named Nationalliga B (NLB).
The season was divided into two phases. The first of which was the qualification phase, and within the divisions the teams played a double round-robin, one at home and the other away. The divisions were then split. The first eight teams of the NLA then competed in the championship group and played a further double round-robin. The teams in ninth to twelfth position competed with the top four teams of the NLB in a promotion/relegation group to decide which two teams would play top tier next season. The top tier was to be reduced from twelve to ten clubs, the second tier was to increased to 16 teams. The other eight teams of the second tier competed in a play-out against relegation to the 1. Liga. Originally two teams were to be relegated.
This was to be the last season that the Swiss championship would be competed in this format. For the next season the Nationalliga would be renamed to Swiss Football League and the championships were subsequently renamed as Super League and Challenge League.
The first eight teams of the qualification then competed in the Championship Round. They took half of the points (rounded up to complete units) gained in the Qualification as bonus with them.