Viby, often referred to as Viby Sjælland to distinguish it from Viby J in Jutland, is a satellite and commuter town to Roskilde and Copenhagen, located 8 km south of Roskilde, in Roskilde Municipality, some 30 km west of Copenhagen, Denmark. The original village, now Gammel Viby, is located in the northern part of town while the modern district has formed around Viby railway station which opened on the Danish Main Line between Copenhagen and western Denmark in 1854. The village of Dåstrup is now also an integrated part of Viby. As of 1 January 2023, Viby had a population of 4,814.

Viby J is a former town and now a district, in the southwestern part of Aarhus in Denmark. The district has almost 30,000 inhabitants. The "J" stands for Jutland, as there is another town called Viby on the island of Zealand, officially referred to as Viby Sj and also a village on Funen.