| Competition: | Serie B TIM |
| Season: | 2003–04 |
| Promoted: | Palermo (3rd title) Cagliari Messina Livorno Atalanta Fiorentina |
| Relegated: | Como Avellino Napoli |
| League Topscorer: | Luca Toni (30 goals) |
| Matches: | 552 |
| Total Goals: | 1287 |
| Prevseason: | 2002–03 |
| Nextseason: | 2004–05 |
The 2003–04 Serie B is the 72nd season since its establishment in 1929. It is the second highest football league in Italy.
This 46 matchdays championship was the longest tournament in all the history of the Italian football.
Treviso, Avellino, AlbinoLeffe and Pescara had been promoted from Serie C, while Atalanta, Piacenza, Como, and Torino had been relegated from Serie A. Following the Caso Catania, Fiorentina was added.
| Team | Manager | Kit manufacturer | Shirt sponsor | |
|---|---|---|---|---|
| AlbinoLeffe |  Elio Gustinetti | Legea | SCAME | |
| Ascoli | Aldo Ammazzalorso | Uhlsport | Carisap | |
| Atalanta | Andrea Mandorlini | Asics | Promatech | |
| Avellino |  Zdeněk Zeman | Devis | NewNet Gruppo Tufano | |
| Bari | Giuseppe Pillon | Lotto | Pasta Ambra | |
| Cagliari | Edoardo Reja | A-Line | Terra Sarda | |
| Catania | Nedo Sonetti | Galex | Toyota | |
| Como | Roberto Galia | Erreà | Integratori Integra Sport | |
| Fiorentina | Emiliano Mondonico | Adidas | Fondiaria-Sai | |
| Genoa | Luigi De Canio | Erreà | Costa Cruises | |
| Hellas Verona | Sergio Maddè | Legea | Clerman | |
| Livorno | Walter Mazzarri | Asics | Gruppo Banca Carige | |
| Messina | Bortolo Mutti | Legea | Confcommercio/Caffè Miscela d'Oro, Confcommercio | |
| Napoli | Luigi Simoni | Legea | Russo di Cicciano | |
| Palermo | Francesco Guidolin | Lotto | Provincia di Palermo, Regione Siciliana | |
| Pescara | Cetteo Di Mascio | Ennedue | Saquella Caffè | |
| Piacenza | Luigi Cagni | Lotto | UNICEF | |
| Salernitana | Stefano Pioli | Devis | Centrale del Latte di Salerno | |
| Ternana | Bruno Bolchi | Erreà | EnerTAD | |
| Torino | Ezio Rossi | Asics | Bavaria | |
| Treviso | Adriano Buffoni | Lotto | Segafredo, Provincia di Treviso | |
| Triestina | Attilio Tesser | Asics | San Marco Imaging | |
| Venezia | Angelo Gregucci | Sportika | Gioco Calcio | |
| Vicenza | Giuseppe Iachini | Biemme | Caffè Vero |
Following the Caso Catania, the league had been expanded to 24 clubs, and six special promotions were decided to descend to 22.
Relegation play-offs were also introduced.
Fiorentina had to play a qualification match with 15th-placed team of Serie A, Perugia.
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A.C. Perugia relegated to Serie B, while ACF Fiorentina was promoted to Serie A.
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A.S. Bari was relegated but was later readmitted in place of S.S.C. Napoli.