The Baïse (pronounced [baiːz]) (Occitan: Baïsa) is a 188-kilometre (117 mi) long river in south-western France, left tributary of the Garonne. Its source is in the foothills of the Pyrenees, near Lannemezan. It flows north through the following départements and towns:

• Hautes-Pyrénées: Lannemezan, Trie-sur-Baïse
• Gers: Mirande, Castéra-Verduzan, Valence-sur-Baïse, Condom
• Lot-et-Garonne: Nérac, Lavardac

It flows into the Garonne near Aiguillon.

Tributaries

• Gélise (in Lavardac)
  • Osse (in Nérac)
• Petite Baïse (in L'Isle-de-Noé)