Four boats were badly damaged by fire on Teddington Lock.
A passer-by called 999 after seeing a number of boats alight.
Two boats on the south side of the river were badly damaged by fire and sank. A further two boats on the north side of the river were also damaged by fire.
There were no evacuations and no reports of any injuries.
Four fire engines and around 25 firefighters were called at 0132 and the fire was under control by 0358. Fire crews from Kingston and Surbiton fire stations attended the incident. Teddington life boat also assisted fire crews at the scene.
The cause of the fire is under investigation.