Sauroposeidon proteles, a new brachiosaurid sauropod, is represented by an articulated series of four mid-cervical vertebrae recovered from the Antlers Formation (Aptian–Albian) of southeastern Oklahoma. Most Early Cretaceous North American sauropod material has been referred to Pleurocoelus, a genus which is largely represented by juvenile material and is not well understood. Regardless of the status and affinities of Pleurocoelus, the new taxon is morphologically and proportionally distinct. Among well-known sauropod taxa, Sauroposeidon is most similar to Brachiosaurus; particularly noteworthy are the neural spines, which are set forward on the centra and are not bifurcate, and the extremely elongate cervical ribs. Sauroposeidon and Brachiosaurus also share a derived pattern of pneumatic vertebral ultrastructure and a mid-cervical transition point, at which neural spine morphology changes from very low (anteriorly) to very high (posteriorly). Autapomorphies of Sauroposeidon include posterior placement of the diapophyses, hypertrophied pneumatic fossae in the lateral faces of the neural spines and centra, and an extraordinary degree of vertebral elongation (e.g., C8 = 1.25 m; 25% longer than Brachiosaurus). Additional sauropod material from the Early Cretaceous Cloverly Formation may be referrable to the new Oklahoma sauropod, which appears to be the last of the giant North American sauropods and represents the culmination of brachiosaurid trends towards lengthening and lightening of the neck.