Sisu K-50SS is a six-wheel-driven ballast tractor made by the Finnish heavy vehicle producer Suomen Autoteollisuus (SAT).

[2] Only one unit was produced in 1961 and it is the largest roadworthy motor vehicle ever built in the Nordic countries.

[1] SAT had previously produced six heavy Sisu K-36 earthmovers and the know-how was utilised in the engineering work.

Its 223-kW (300 hp) output is 43% and 1 070 Nm torque 56% higher compared to the standard naturally aspirated type.

[2] The K-50SS was used for heavy hauling in North Finnish hydroelectric power plant construction sites.

[2] The K-50SS is the all-time largest individual roadworthy automobile unit produced in the Nordic countries, without a tipper or other superstructures.

It was restored to its original outfit little by little and presented in various vintage vehicle meetings,[1] until Kemijoki Oy gave the Sisu to Oulu Automobile Museum in 2006.