The Gélise (pronounced [ʒeliːz]) is a tributary of the river Baïse in Gascony, southwestern France. It is 92.0 km (57.2 mi) long.

The name Gélise comes from the Aquitanian hydronymic root Jel, meaning "watercourse."

The Gélise originates in the Gers département at Cahuzères, north of Lupiac, then it flows northwest in the direction of Eauze. It drains the land around Castelnau-d'Auzan, then it flows northeast where it forms the natural boundary between the forest of the Landes of Gascony, the slopes of the Armagnac-Ténarèze.

It unites with the Baïse just after passing the fortified mill of Lavardac in Lot-et-Garonne.

The Osse, a right tributary, joins the Gélise near Nérac. The Auzoue, a right tributary, joins the Gélise at Mézin. The Izaute, a right tributary, joins the Gélise at Saint-Pé-Saint-Simon.

The Gélise passes through the following départements and cities:

• Gers: Dému, Eauze
• Lot-et-Garonne: Sos, Poudenas, Mézin, Barbaste

• The Gélise (Natura 2000) (in French)