The Jerma (Serbian Cyrillic: Јерма) or Erma (Bulgarian: Ерма) is a river in southeastern Serbia and western Bulgaria.

Starting from the area between the artificial Lake Vlasina and the Bulgarian border, it flows to the northwest on the eastern slopes of the Gramada mountain, passing through the village of Klisura, after which it enters the area of Знепоље (Znepolje, Bulgarian: Знеполе, Znepole), an arid region stretching over the border into Bulgaria.

Continuing through the Bulgarian valley of Znepole curving around the eastern side of the Ruy mountain, the river, now called the Erma, is also known as Transka reka after the town of Tran, a regional centre of this area.

The Erma receives its major tributary, the Yablanitsa, right before it re-enters Serbia after a 26 kilometres (16 mi) travel through Bulgaria.

Proceeding between the mountains of Greben and Vlaška planina, it runs close to the villages of Trnsko-Odorovce and Vlasi, and the monasteries of Sveti Jovan, Sveti Nikolaj and Sveta Bogorodica, before it empties into the Nišava River, southeast of Pirot after a total of 48 km (30 mi) in Serbia (thus belonging to the Black Sea drainage area).