| State: | LA |
| Type: | LA 1955 |
| Route: | 646 |
| Map Custom: | yes |
| Map Notes: | LA 646 highlighted in red on a modern map |
| Length Mi: | 3.3 |
| Established: | 1955 renumbering |
| Deleted: | 1975 |
| Direction A: | Southeast |
| Terminus A: | west of Thibodaux |
| Direction B: | Northwest |
| Terminus B: | End state maintenance at Lafourche-Assumption parish line |
| Parishes: | Lafourche |
| Previous Type: | LA |
| Previous Route: | 645 |
| Next Type: | LA 1955 |
| Next Route: | 647 |
Louisiana Highway 646 (LA 646) was a state highway that served Lafourche Parish. It spanned a total of 3.3miles along French Plantation Road and Little Choupic Road west of Thibodaux, near the Lafourche-Assumption parish line.
From the southeast, LA 646 began at an intersection with LA 308 west of Thibodaux and an area known as Laurel Grove. It proceeded northeast along French Plantation Road then turned in a general westerly direction to zigzag along Little Choupic Road to its terminus at the Assumption Parish line.
LA 646 was an undivided, two-lane highway for its entire length.
LA 646 was designated as State Route C-1338 prior to the 1955 Louisiana Highway renumbering.