The Humber-class monitors were three large gunboats under construction for the Brazilian Navy in Britain in 1913. Designed for service on the Amazon River, the ships were of shallow draft and heavy armament and were ideally suited to inshore, riverine and coastal work but unsuitable for service at sea, where their weight and light draft reduced their speed from a projected twelve knots to under four. The class comprised Humber, Mersey and Severn. All three were taken over by the Royal Navy shortly before the outbreak of the First World War and were commissioned as small monitors. All three saw extensive service during the war and were sold in 1919.

A monitor is a relatively small warship that is neither fast nor strongly armored but carries disproportionately large guns. They were used by some navies from the 1860s, during the First World War and with limited use in the Second World War.