Cordillera Oriental, Mountain range in eastern Dominican Republic.

The Cordillera Oriental, also called Sierra de El Seibo, is a range of moderate hills in eastern Dominican Republic, running about 80 km from Los Haitises to Miches. Most hills stay under 200 m, with Loma en Pequito the highest at 879 m, followed by Loma Vieja at 736 m. Its karst limestone south slope holds many caves.

The Cordillera Oriental is a Cretaceous formation, older than the Miocene karst of Los Haitises, which lies within it. Its south slope carries a long limestone piedmont riddled with caves, where rainfall drains underground rather than in surface rivers. The range runs west to east from Cotuí, in Sánchez Ramírez, through Monte Plata, Hato Mayor and El Seibo to La Altagracia.

The range crosses Monte Plata, Hato Mayor, El Seibo and La Altagracia provinces, where the land is given over mainly to cattle raising on the south and east slopes. The name Sierra de El Seibo is still used for the chain. In Los Haitises, rain is heavy but no rivers run on the surface; drainage goes underground through the limestone.

The range runs about 80 km from the Los Haitises area to Miches, separated from the sea by the Sabana de la Mar plain, with the Caribbean coastal plain to the south. The karst south slope holds numerous caves, and in Los Haitises rain drains underground, so no surface rivers appear. Annual rainfall of 2,000 to 3,000 mm and an average of 18 °C make waterproof gear useful.

The range's highest point is Loma en Pequito at 879 m, not the 2,400 m once claimed; Loma Vieja reaches 736 m. Most of the chain is hills under 200 m. Geologically it is Cretaceous limestone, older than the Miocene Los Haitises karst beside it.