The Pat River (Rivière à Pat) is a tributary on the east bank of the Nicolet Southwest River. It crosses the municipalities of Sainte-Élizabeth-de-Warwick, Sainte-Séraphine and Sainte-Clotilde-de-Horton in the Arthabaska Regional County Municipality (MRC), in the administrative region of Estrie, in Quebec, in Canada.

The main neighboring hydrographic slopes of the Pat river are:

• north side: Nicolet Southwest River, Nicolet River;
• east side: Nicolet River, rivière des Rosiers;
• south side: Léon-Gélinas stream, Dubuc stream, Nicolet Southwest River;
• west side: Nicolet Southwest River.

The Rivière à Pat originates from various streams in the municipality of Sainte-Élizabeth-de-Warwick, very close to the limit of the municipality of Kingsey Falls.

From its source, Pat's River flows on 18.4 kilometres (11.4 mi) in the following segments:

• 3.7 kilometres (2.3 mi) north-west, in the municipality of Sainte-Élizabeth-de-Warwick, to the intermunicipal limit of Sainte-Séraphine;
• 4.8 kilometres (3.0 mi) north, passing 2.2 kilometres (1.4 mi) east of the village of Sainte-Séraphine, up to the road to seventh rank;
• 4.8 kilometres (3.0 mi) north-west, up to the municipal limit of Sainte-Clotilde-de-Horton;
• 5.1 kilometres (3.2 mi) westward to its confluence, of which 2.8 kilometres (1.7 mi) in Sainte-Séraphine, 2.0 kilometres (1.2 mi) in Sainte-Clotilde-de-Horton, 0.6 kilometres (0.37 mi) in Sainte-Séraphine and 1.7 kilometres (1.1 mi) in Sainte-Clotilde-de-Horton.

The Rivière à Pat drains on the east bank of the Nicolet Southwest River. Its confluence is located 1.6 kilometres (0.99 mi) upstream from Île Lemire and upstream from the village of Sainte-Clotilde-de-Horton.

The toponym "Rivière à Pat" was formalized on September 5, 1985, at the Commission de toponymie du Québec.

• Lake Saint-Pierre
• List of rivers of Quebec