| River Lett | |
| Subdivision Type1: | Country |
| Subdivision Name1: | Australia |
| Subdivision Type2: | State |
| Subdivision Name2: | New South Wales |
| Subdivision Type3: | Region |
| Subdivision Name3: | Sydney Basin (IBRA), Central Tablelands |
| Subdivision Type5: | Municipality |
| Subdivision Name5: | City of Lithgow |
| Length: | 10km (10miles) |
| Source1: | Bell Range |
| Source1 Location: | near |
| Mouth: | confluence with the Coxs River |
| Mouth Location: | at Glenroy, south-west of |
| River System: | Hawkesbury-Nepean catchment |
| Tributaries Left: | Dargans Creek |
| Tributaries Right: | Kangaroo Corner Creek |
| Custom Label: | National park |
The River Lett is a perennial stream of the Hawkesbury-Nepean catchment. It is located in the Central Tablelands region of New South Wales, Australia.
The River Lett rises on the southern slopes of the Bell Range, below, approximately north of, and flows generally west south-west, joined by two minor tributaries, before reaching its confluence with the Coxs River, at Glenroy, south-west of the historical settlement of . The river course is approximately .
The Great Western Highway traverses the River Lett near Hartley, at the bottom of the ascent of River Lett Hill.[1] The remains of an old timber bridge that carried traffic on the Great Western Highway over the River Lett can be found near the settlement of Hartley.[2]