The Sulm (Austrian German: [sʊlm]) is a river in Southern Styria, Austria. It is 29.3 km (18.2 mi) long (66.0 km (41.0 mi) including its longer source river Schwarze Sulm [ceb; de; sv]). Its drainage basin is 1,121 km² (433 sq mi). Its two source rivers Schwarze and Weiße Sulm [ceb; de; sv] both originate at the eastern slopes of the Koralpe (a north-south running mountain range in the Southeastern Alps which separates Styria from Carinthia). It flows eastwards towards the Mur through the districts of Deutschlandsberg and Leibnitz. The Sulm valley runs from the Western Styrian hill ranges to the Eastern Styrian hills and lowlands.

The main tributaries of the Sulm are the Schwarze Sulm ("Black Sulm", 36.7 km (22.8 mi) long, with Schwanberg as the central market town) and the Weiße Sulm ("White Sulm", 29.3 km (18.2 mi) long, with Wies) which merge near the village of Prarath, upstream of Gleinstätten. It is there where the actual Sulm valley is considered to begin. Further downstream, near Großklein and Fresing, the Sulm proceeds to receive the river Saggau, and - close to Leibnitz - the river Laßnitz, immediately before the Sulm makes a sharp turn to the Southeast and flows almost parallel to the Mur before joining it.

The Sulm runs on top of a thick bed of its own massive quaternary deposits, and therefore the valley bed is now quite flat although (as can be deduced from the remaining terraces on its rims) it must have been steeper initially. The river therefore breached its banks easily, and used to cause frequent major flooding, until it was regulated from the 1960s onward.

A unique geographic feature in the Sulm valley is the Sausal mountain range which has its own mild microclimate, supporting a significant fraction of Styria's wine-growing economy.

The Sulm valley has yielded archeological finds from the Neolithic period onward, illustrating that it has served as an east-west traverse for thousands of years. In the Iron Age, during part of the Hallstatt Culture period, a settlement on the Burgstallkogel between Gleinstätten and Großklein had considerable regional importance. The associated necropolis, part of which can still be seen, is one of the largest and best-preserved in Central Europe although much has been plundered from the late 19th century onward.

The Frauenberg, a hill near Leibnitz, is especially significant in terms of pre-history. Apparently a halidom for female goddesses throughout the Neolithic and Celtic periods, its plateau bore a temple during Roman times when the municipium of Flavia Solva flourished. Recently, one of the largest known cemeteries from the late Roman period has been identified on one of its slopes.

It is believed that the Sulm valley was only minimally (if at all) populated during the Migration Period, after the Romans had withdrawn from the province of Noricum in the 5th century. Records from the 10th and 11th century C.E., when it was resettled by Bavarian emissaries from Salzburg, describe it as swamped and hardly passable.

Many settlements in the lower Sulm valley likely date back to the 9th century when the Bavarians reclaimed the region from the predecessors of the current Slovenes who had loosely settled it after the rule of the Avars had succumbed to Charlemagne. During the 12th century the region became a territory of the church-state of Salzburg, and remained so for more than 400 years. The Khuenburg family, a significant branch of Salzburg nobility, owned large swaths of territory until the late 19th century.