The diamondback terrapin (Malaclemys terrapin) is a brackish-water turtle ranging from Texas to Massachusetts, as well as an isolated population on the island of Bermuda. Louisiana likely holds the most available habitat of any other state for the species (over 650,000 ha of brackish and salt marshes), yet little is known about terrapin distribution and abundance throughout coastal Louisiana. Knowledge is particularly scant in southwestern Louisiana, where only 12 specimen records exist, the most recent record being from 1972. We wanted to determine whether 1) terrapin populations persist in historical collection localities, 2) terrapin populations are present in additional coastal marsh localities, and 3) terrapin abundance differs among sites in southwestern Louisiana. We sampled for diamondback terrapins during 2011–2013 at 16 sites across southwestern Louisiana; all sites were near historical localities or other apparently suitable brackish or salt marsh habitats. We used unbaited fyke nets with 7.6- and 15.2-m leads to capture terrapins and manually searched tidal ponds by airboat. Terrapins (n  =  490) were captured at 13 of 16 sample sites, with terrapin site abundance varying considerably (CPUE mean: 1.06 terrapins per net day; CPUE range: 0–7.06). High terrapin abundance was always associated with large expanses of unimpounded brackish and salt marshes, whereas low abundance was typically associated with smaller marsh sizes, channels, or bayous that no longer connected to the Gulf of Mexico, and the presence of crab traps. All sites with terrapin captures represent either a new locality or the first record for a locality in over 40 yrs. The results of this study underscore the continued need for better distribution and abundance data for poorly studied portions of a species' range, especially those that are of conservation concern.