ABSTRACT The offshore Santos and Campos basins of the southeastern Brazilian margin are currently the focus of extensive hydrocarbon exploration following some of the largest global oil discoveries made within the so-called pre-salt section. It is widely accepted that these basins initially developed during the Neocomian breakup of Gondwana and separation of Africa from South America. However, significant debate exists concerning the regional tectonic significance and timing of Early Cretaceous tholeiitic basalts drilled across the Pelotas, Santos, and Campos basins; the distribution of continental crust; distribution and temporal development of thickened oceanic crust; and basement influence on basin development and timing. We have reviewed earlier published tectonic analyzes in addition to a comprehensive integration of both old and new seismic reflection and refraction data, gravity, and well calibrations to place new constraints on the tectonic evolution of the Santos and Campos basins. Upper and lower crustal refraction velocities and densities across the São Paulo Plateau were once considered to indicate continental basement. Our reinterpretation shows they are equally consistent with thickened oceanic crust (which we term magmatic crust ). We define tectonic domains within the Santos and Campos basins and show that crustal thicknesses across the Outer Basin High and Jupiter Terrace range from 15 to 20 km, whereas the Deep Basin and standard (Penrose) oceanic crust to the east of the Outer Basin High show crustal thicknesses of 1–3 km and 6–8 km, respectively. We interpret the along- and across-strike variations in crustal thickness variations to be a function of the proximity to the Tristan da Cunha plume and its magma budget combined with structural reworking of this thickened oceanic crust by superimposed late-stage extensional faulting. Seismic reflection profiles from the Pelotas, Santos, and Campos basins show the existence of relatively thick oceanic crust characterized by SDR (seaward dipping reflector) geometries that progressively decrease in age to the east and onlap earlier syn-tectonic volcanic flows and/or extended continental basement that form part of a “necking zone.” Analyzing SDRs from the northern Jacuipe Basin demonstrates that they are upper crustal counterparts to Layer 2 of oceanic crust, whereas the lower crust is equivalent to Layer 3 of oceanic crust even exhibiting a crisscross reflectivity pattern possibly related to shear within magma chambers. In particular, we suggest that SDRs represent subaerial seafloor spreading that laterally merges structurally and petrologically into Layer 2 of Penrose oceanic crust. In this interpretation, the first SDR flows onto thinned continental crust are critical because the causative eruptive center defines the location of continental breakup, the timing of breakup, and the initiation of subaerial magmatic spreading. Based on the identification and distribution of SDRs in the Pelotas, Santos, and Campos basins and seismic mapping calibrated with exploration well data, we propose a general template for the structural and stratigraphic development of southeast Brazil that comprises (1) a relatively thick continental crust in the extreme western, proximal part of the margin, (2) deposition of pre-rift and synrift volcanic flows (equivalent to the Paraná basalts) on extended continental crust, (3) a continental crustal necking zone, (4) an exhumation point (i.e., the complete necking of the continental crust) after which post-breakup, magmatic SDR crust onlaps earlier syn-tectonic basement and volcanics, and (5) for the Campos Basin, a second necking zone involving the extensional deformation of magmatic crust. Continental extension is assumed to span Berriasian–late Valanginian (134–145 Ma), consistent with rift basins along the entire eastern Brazilian margin. Lithospheric breakup is considered to be late Valanginian–early Hauterivian (132–134 Ma), triggered by the rapid emplacement of the Paraná Large Igneous Province. As such, deposition east of the continental necking zones is post-breakup (Arutu-, Buracica-, Jiquia-, and Alagoas-aged sediments) on new “real estate” crust. For the Pelotas and southern Santos basins, subaerial magmatic crust was emplaced east of the continental necking zone with the generation of SDRs that progressively decrease in age to the east. In the northern Santos Basin, post-breakup magmatic crust is emplaced east of the continental necking zone, but SDR geometries are not observed. For the Campos Basin, subaerial seafloor spreading forms oceanic crust east of the continental necking zone, and later, seaward of the magmatic necking zone. This second phase of spreading is characterized by additional SDRs. A time-transgressive distribution of magmatic basement age is implied, with older crust emplaced along the continental necking zone and younger crust to the east; depocenter migration is evidenced by the shift in the easterly limit of Buracia-, Jiquía-, and Alagoas-aged sedimentation. In places, ridge jumps may be superimposed (e.g., Abimael Ridge), which locally reverses this age progression. Although several authors have previously suggested that SDRs may represent subaerial seafloor spreading, this is the first analysis to provide an integrated and coherent, self-consistent tectonic analysis of SDRs that defines the location and timing of continental breakup, the initiation of seafloor spreading, the transition to Penrose oceanic crust, and the timing of margin flooding.