Tyrifjorden (Lake Tyri) is Norway's fifth largest lake with an area of 137 km². It has a volume of 13 km³, is 295 meters deep at its deepest, and lies 63 meters above sea level. The lake's primary source is the Begna river, which discharges into Tyrifjorden at Hønefoss where the river forms the waterfall of Hønefossen. Its primary outlet is at Geithus near the lake's southwest corner, where Tyrifjorden discharges into the Drammenselva river.

Tyrifjorden is located in the county of Buskerud and borders the municipalities of Hole, Lier, Modum, and Ringerike. Tyrifjorden is a landlocked fjord. It consists of a main body, Storfjorden, along with the Holsfjorden, Nordfjorden, and Steinsfjorden branches.

The Old Norse form of the name was just Tyri (or Tyrvi). This uncompounded name is also the first element in the name Tyristrand. The name is derived from the word tyri meaning "old/dead pine (wood)", referring specifically to the woods of the western side of the lake. The last element -fjorden (the finite form of fjord) is a later addition.