Thoracic epaxial muscles of diplodocid and dicraeosaurid sauropods are reconstructed for the first time using an Extant Phylogenetic Bracket approach. In the dorsal vertebral column, three different epaxial muscle groups were present. The medialmost m. transversospinalis and the laterally adjacent m. longissimus dorsi were connected to the neural arches. The lateralmost m. iliocostalis was connected to the dorsal ribs. The medial part of m. transversospinalis of diplodocids and dicraeosaurids comprised at least two tendon systems that had a trellis-like arrangement. Osteological characters of the dorsal vertebrae in related taxa suggest a similar configuration of the thoracic epaxial muscles in all eusauropods, although there was variation in tendon arrangements and in the cross-section of each muscle. The thoracic epaxial musculature in eusauropods played an important role for trunk support; its variation in different eusauropods is directly connected with bony support structures and influenced the mobility of the trunk and locomotion. Modifications in the thoracic epaxial musculature are especially apparent in Saltasauridae, which suggests differences in their locomotor capabilities, such as increased trunk mobility or larger stride lengths.