A geophysical transect across the central Gulf of Mexico coastal plain shows that the early Paleozoic continental margin of southern Laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late Paleozoic Ouachita orogeny and formation of the modern Gulf of Mexico coastal plain. The thickness of the crystalline crust decreases abruptly across the margin over a distance of ∼50 km, from 35 km beneath the Black Warrior foreland basin to 10 km beneath the Ouachita fold-and-thrust belt. This abrupt decrease in crustal thickness is similar to modern transform margins, but very different from most rifted margins, which display much more gradual transitions in crustal thickness. The geophysical data indicate an absence of synrift intrusive and volcanic rocks, underplated mafic rocks at the base of the crust, and abnormally thick oceanic crust adjacent to the margin. The lack of these features is also characteristic of modern transform margins. Combined with transects across the margin farther west, the data confirm previous suggestions that the central Gulf of Mexico coastal plain overlies an ∼800-km-long transform segment of the late Proterozoic–early Paleozoic southern Laurentian continental margin that extends continuously from western Arkansas to southeast Alabama.