River monitor was the strongest class of riverine warships. The name originally goes from USS Monitor and sea monitor warship type.

River monitors were used on rivers and lakes. They were armed with several guns caliber 75 mm – 130 mm and machineguns. Usually they had small draught, but their displacement, size and draught varied, depending where they were used. Majority of river monitors had displacement 250-500 ton (e.g. used on the Danube). Most river monitors were lightly armoured, some had heavier armour.

River monitors were used mainly during World War I and World War II. On the Danube, the monitors were employed by Austria, Hungary, Romania and Yugoslavia. On Asian rivers, first of all Amur, USSR and Japan used big river monitors, up to 1000 ton displacement, armed with 130 mm guns. Smaller monitors (70-100 ton displacement) were used by Poland in 1939 and by the USSR in 1941 on the Pripyat River.

During Vietnam war, the USA used smaller gunboats called monitors on the Mekong.

See also: Monitor warship type, USS Monitor