The Alz (German: [alt͡s] ^ⓘ) is a river in Bavaria, southern Germany, the only discharge of the Chiemsee. Its origin is on the northern shore near Seebruck. It is a right tributary of the Inn, into which it flows in Marktl. Other towns on the Alz are Altenmarkt an der Alz, Trostberg, Garching an der Alz and Burgkirchen an der Alz.

The Alz is divided into the Obere Alz (upper Alz) and the Untere Alz (lower Alz). The section from the Chiemsee down to Altenmarkt is called the Obere Alz. From Altenmarkt to the mouth in Marktl, it is called the Untere Alz.

The Traun, a 29 km (18 mi) river flowing past the regional administrative center of Traunstein, is a tributary of the Alz.

From pre-indoeuropean *alz "swamp, alder".^{[citation needed]}