The Municipality of Miren-Kostanjevica (pronounced [ˈmiːɾən kɔˈstaːnjɛʋitsa]; Slovene: Občina Miren - Kostanjevica, Italian: Comune di Merna Castagnevizza) is a municipality in western Slovenia, on the border with Italy.

Miren-Kostanjevica is part of the Goriška region of the Slovene Littoral, and its main settlements are Miren (the seat of the municipality) and Kostanjevica na Krasu.

[2] Historically, the area was connected with the neighbouring village of Savogna d'Isonzo (Slovenian: Sovodnje ob Soči), which was left in Italy after the Paris Peace Treaty of February 1947.

Kostanjevica na Krasu, on the other hand, is a center of a larger underpopulated rural area located on the high Karst Plateau.

In the local self-government reform of 1994, the two centers united to form a single municipality.