Transform marginal plateaus (TMPs) are submarine seafloor highs located at the continental slope, often at the boundary of two ocean basins of different ages and associated to at least one transform or highly oblique margin. The systematic study of TMPs can, therefore, answer questions about rifting and continental margin development. The Demerara TMP (offshore Suriname and French Guiana) is located at the border between the Central Atlantic, which opened during the Lower Jurassic and the Equatorial Atlantic, which opened during the Lower Cretaceous. This study, based on wide-angle seismic data modeling from the northern and western section of the Demerara Plateau, provides information on both the lower volcanic unit of this TMP and the adjacent oceanic crust. The results confirm that the crust of the Demerara Plateau is around 30 km thick and consists of lava flows possibly mixed with crust of continental origin in its deeper layers. Seismic velocities (exceeding 7 km/s) are compatible with those of volcanic oceanic plateaus. To the west, a relatively wide transition zone separates the plateau from the Jurassic oceanic crust, which is composed of two layers, and is much thicker than normal oceanic crust (~11 km). During the Cretaceous, the plateau was sharply cut by transform and highly oblique structures, separating the Demerara Plateau from its transform conjugate, the Guinea Plateau. As a result, the Demerara Plateau is flanked to the north by a magma-poor/strongly tectonized Cretaceous oceanic domain with thin (2–3 km) crust, likely partially consisting of serpentinized mantle. In contrast, the oceanic crust located towards the south-east appears to be more characteristic of typical oceanic crust in composition though slightly thinner than normal (4–6 km) thickness. Our analysis allows us to propose a new 3D vision of the crustal structure of the Demerara TMP and its borders.