| State: | LA |
| Type: | LA |
| Route: | 18 |
| Length Mi: | 79.7 |
| Map: | LA 18 map.svg |
| Established: | 1955 renumbering |
| Direction A: | West |
| Terminus A: | in Donaldsonville |
| Junction: | in North Vacherie in Wallace in Edgard in Luling in Bridge City in Marrero in Harvey |
| Direction B: | East |
| Terminus B: | in Gretna |
| Parishes: | Ascension, St. James, St. John the Baptist, St. Charles, Jefferson |
| Previous Type: | LA |
| Previous Route: | 17 |
| Next Type: | LA |
| Next Route: | 19 |
Louisiana Highway 18 (LA 18) is a state highway that serves Ascension, St. James, St. John the Baptist, St. Charles, and Jefferson Parishes. Called the Great River Road, it runs from west to east, parallel to the west bank of the Mississippi River, running from Donaldsonville to Gretna. It spans a total of 79.7miles. In the more rural parts of LA 18's span, it is commonly referred to as River Road, but it becomes 4th Street once it enters Westwego.
LA 18 begins as an undivided, two lane road in downtown Donaldsonville as Bayou Road, which runs parallel to the Mississippi River. It intersects with the Sunshine Bridge (LA 70) east of Donaldsonville, the Veterans Memorial Bridge (LA 3213) in Wallace, and Interstate 310 near Luling as it continues downriver. At Bridge City, LA 18 leaves the river and briefly merges with U.S. Highway 90 northbound, then turns east along Seven Oaks Blvd. while U.S. 90 crosses the Mississippi River via the Huey P. Long Bridge. LA 18 meets and briefly cojoins the alternate river road, LA 541. When LA 18 enters Westwego, LA 18 becomes 4th Street but still continues parallel to the river. In Gretna, LA 18 turns south, away from the river: first onto Huey P. Long Avenue for one block, then east along 5th Street to Lafayette Street. LA 18 then goes south and ends at the intersection of Lafayette St. and the Westbank Expressway.
Between the junction with LA 52 in Luling and Huey P. Long Avenue in downtown Gretna, LA 18 is largely the original routing of U.S. Highway 90 (also LA 2 before the 1955 renumbering) and before that, the Old Spanish Trail highway. (One notable exception is that LA 18 is routed along Louisiana Street rather than the parallel Sala Avenue through Westwego's historic district.)
| State: | LA |
| Type: | LA-Spur |
| Route: | 18 |
| Location: | Westwego |
| Length Mi: | 0.62 |
| Formed: | 1960s |
In Westwego, LA 18 turns from Louisiana Street onto 4th Street. LA 18 Spur continues southward for 0.6miles on Louisiana Street until it meets the Westbank Expressway (U.S. 90 Business).