Viðoy covers an area of 41.01 km² and is the second largest island in Norðoyggjar. There are two villages on the island, Viðareiði in the northern part and Hvannasund in the southwestern part of the island.
The highest point on Viðoy is Villingadalsfjall north of Viðareiði, which at 841 m is the third highest mountain in the Faroe Islands. To the far north is the vertical cliff Enniberg, whose peak is 754 m above sea level In the middle of the island lies the characteristic pyramid-shaped Malinsfjall at 750 m.
Viðoy is divided into two municipalities, Viðareiðis Kommuna and Hvannasunds Kommuna. Hvannasunds Kommuna also includes villages on Borðoy on the other side of the sound.