Sundbyfoss is a village in Holmestrand Municipality in Vestfold county, Norway. The village is located along the lake Hillestadvannet, about south of the village of Hof. The village of Gullhaug lies about to the southeast and the town of Holmestrand lies about to the southeast. The village of Hvittingfoss in Kongsberg Municipality lies about to the west. Hillestad Church lies about east of the village.
The village has a population (2024) of 562 and a population density of .
Sundbyfoss is named after the local waterfall on the river Sundbyelva. The waterfall provided the basis for early industry in the village. Today, Sundbyfoss is mainly a residential area. The largest industrial enterprise is the "Haslestad Bruk" sawmill. Historically, the old private TønsbergâÂÂEidsfoss railway line and HolmestrandâÂÂVittingfoss railway line both went through Sundbyfoss.