Rostrenen (French pronunciation: [ʁɔstʁənɛ̃]; Breton: Rostrenenn) is a commune in the Côtes-d'Armor department and Brittany region of northwestern France.

Rostrenen is border by Mellionnec to the south, Plouguernevel to the east, Kergrist-Moëlou to the north, and Glomel to the west.

The average annual rainfall is 1,145.7 mm (45.11 in) with January as the wettest month.

The temperatures are highest on average in August, at around 17.0 °C (62.6 °F), and lowest in January, at around 5.2 °C (41.4 °F).

On 13 September 2004 the municipality launched a linguistic plan as part of the Ya d'ar brezhoneg (Yes to Breton) campaign.