Coordinates: 64°40′N 37°30′E / 64.667°N 37.500°E
The Onega Peninsula is located in Arkhangelsk Oblast, Russia. It protrudes into the White Sea, with Onega Bay to the south-west, and Dvina Bay to the north-east. The length of the peninsula is about 150 kilometres (93 mi), and the width varies between 60 kilometres (37 mi) and 75 kilometres (47 mi).
The coastal area of the peninsula is populated. The villages (counterclockwise) of Nyonoksa, Syuzma, Krasnaya Gora, Pertominsk, Unsky, Yarenga, Lopshenga, Letny Navolok, Letnyaya Zolotitsa, Pushlakhta, Lyamtsa, Purnema, and Nizhmozero are all located at or near the coast. The north-eastern coast of the peninsula is crossed by the Una Bay, a gulf in the Dvina Bay, which is about 20 kilometres (12 mi) long. The villages of Una and Luda are located at the inner coast of the Una Bay. The interior of the peninsula (north-west of the line connecting Luda and Purnema) is a wilderness without any permanent population. There is some logging at Verkhneozersk, south-east of Luda. The interior of the peninsula is hilly, and there are many lakes, the biggest of which are Lake Myandozero, Lake Vezhmozero (both lakes drain to the Vezhma River - a tributary of the Una Bay), Lake Lyamitskoye (which drains to the Lyamtsa River, flowing west), and Lake Bolshoye Vygozero (which drains into the Zolotitsa River, also flowing west).
Administratively, the peninsula is shared between Onezhsky and Primorsky Districts and the city of Severodvinsk of Arkhangelsk Oblast. On 17 December 1940 Belomorsky District was established on the peninsula, on the areas which previously were parts of Primorsky and Onezhsky Districts. The administrative center of the district became the selo of Pertominsk. On 30 September 1958 Belomorsky District was abolished, and the area of the district was shared between Primorsky and Onezhsky Districts.