"Amargasaurus" was small for a sauropod, measuring in length and weighing approximately . It followed the typical sauropod body plan, with a long tail and neck, a small head, and a barrel-shaped trunk supported by four column-like legs. The neck of "Amargasaurus" was shorter than in most other sauropods, a common trait within the Dicraeosauridae. Measuring in length, the neck corresponded to 136% of the length of the dorsal vertebral column. This is comparable to "Dicraeosaurus" (123%) but greater than in the extremely short-necked form "Brachytrachelopan" (75%). The neck consisted of thirteen cervical vertebrae, which were opisthocoelous (convex at the front and hollow at the back), forming ball-and-socket joints with neighboring vertebrae. The trunk was made out of nine dorsal and probably five fused sacral vertebrae. The foremost dorsals were opisthocoelous, while the remaining dorsals were amphyplatyan (flat on both ends). Robust transverse processes (lateral projections connecting to the ribs) indicate a strongly developed rib cage. The dorsal vertebrae of "Amargasaurus" and other dicraeosaurids lacked pleurocoels, the deep lateral excavations that were characteristic for other sauropods.