Rostrenen (; ) is a commune in the French department of Côtes-d'Armor, in Brittany.
Rostrenen is border by Mellionnec to the south, Plouguernevel to the east, Kergrist-Moëlou to the north, and Glomel to the west.
Rostrenen has an oceanic climate (Köppen climate classification Cfb). The average annual temperature in Rostrenen is . The average annual rainfall is with December as the wettest month. The temperatures are highest on average in August, at around , and lowest in January, at around . The highest temperature ever recorded in Rostrenen was on 18 July 2022; the coldest temperature ever recorded was on 20 January 1963.
In French the inhabitants of Rostrenen are known as Rostrenois.
On 13 September 2004 the municipality launched a linguistic plan as part of the Ya d'ar brezhoneg (Yes to Breton) campaign. In 2008, 34.5% of primary children attended bilingual schools.