River Allen, Truro
- Not to be confused with River Allen, Cornwall
The River Allen runs to the north of Truro and eventually joins the Truro River in the city.
The name is from the Cornish Dowr Alen, meaning 'shining river', and it is sometimes called the 'St Allen River'. It is one of two rivers in Cornwall which share this name, the other, the Allen in the north of the County, is a major tributary of the River Camel.
The Truro Allen rises at Ventoneage north of St Allen (50°18’54"N, 5°3’29"W) and flows southwards through the Idless Valley into Truro. Here it joins the River Kenwyn to form the Truro River.