Studying brain evolution in extinct taxa can be challenging due to a lack of close correspondence between the morphology of the endocranial cavity and that of the underlying brain. Cranial endocasts may be faithful brain proxies in certain groups (mammals, birds) due to relatively complete filling of the cavity with neural tissue in life. However, the brain does not fill the endocranial cavity in many adult non‐avian archosaurs, making their endocasts less reliable indicators of brain size and shape. As such, previous studies of relative brain size and evolution in archosaurs relied on untested assumptions about brain‐endocast fidelity. We propose a new approach known as Gross Anatomical Brain Region Approximation (GABRA), which involves importing a digital endocast, derived from CT scanning and 3D visualization software, into modeling software (Maya). In Maya, brain regions underlying the endocast are modeled as 3D polygons. Discernable osteological correlates for soft‐tissue structures (e.g., neurovascular canals, dural sinuses, fossae formed by the brain itself) present on the endocast serve as anatomical landmarks that inform the location and size of the modeled general brain regions (e.g., cerebral hemispheres, cerebellum, optic lobes, olfactory bulbs). Together, the anatomical landmarks form a set of explicit criteria used to assess endocasts and model brain regions. GABRA criteria and resulting brain models were validated in extant diapsids (lizards, snakes, alligators, birds) via literature review, gross dissection, CT scanning of iodine‐stained specimens, and MRI studies. Therefore, GABRA models produced for extinct archosaurs are credible. Credible GABRA models for extinct dinosaurs provided volumetric estimates for general brain regions, which, when summed, offer a whole‐brain estimate. Such data from several dinosaurs have permitted analyses using modern comparative methods of relative brain‐size evolution (e.g., encephalization quotient), with results indicating that brain sizes for many dinosaurs were previously underestimated. Additionally, analyses of GABRA data show a mix of concerted and mosaic patterns of brain evolution, wherein the pituitary and olfactory bulbs emerge as evolving independently from the rest of the brain. In sum, GABRA provides insight into how brains evolved across dinosaur lineages. Future studies will examine within‐lineage changes, offering greater insight into how and why dinosaur brains evolved.Support or Funding InformationFunding for this project was provided by the follow sources: the National Science Foundation, Jurassic Foundation, Sigma Xi Grant‐in‐Aid of Research, Ohio Center for Ecology and Evolutionary Studies, Ohio University Student Enhancement Award, Ohio University Original Work Grant.