Sundbyfoss is a village in Holmestrand Municipality in Vestfold county, Norway. The village is located along the lake Hillestadvannet, about 3 kilometres (1.9 mi) south of the village of Hof. The village of Gullhaug lies about 7 kilometres (4.3 mi) to the southeast and the town of Holmestrand lies about 10 kilometres (6.2 mi) to the southeast. The village of Hvittingfoss in Kongsberg Municipality lies about 8 kilometres (5.0 mi) to the west. Hillestad Church lies about 3 kilometres (1.9 mi) east of the village.

The 0.47-square-kilometre (120-acre) village has a population (2024) of 562 and a population density of 1,195 inhabitants per square kilometre (3,100/sq mi).

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 Tønsberg–Eidsfoss railway line and Holmestrand–Vittingfoss railway line both went through Sundbyfoss.