Postcranial pneumaticity is interpreted as a weight saving adaptation in sauropod dinosaurs, especially in the vertebral column. In some derived sauropods pneumatic features also occur on vertebral ribs. While pneumatic ribs are considered diagnostic of the clade Titanosauriformes, they are also infrequently found in diplodocid sauropods. Here, we describe a partial dorsal rib IV or V referable to Apatosaurus sp. that exhibits a series of superficial pneumatic fossae along its posterior surface. These fossae differ from the morphology found in other pneumatic dorsal ribs of diplodocids, including other apatosaurines and Supersaurus. Moreover, the pneumatic features of this rib are more distally located from the capitulum and tuberculum than in other diplodocids and titanosaurs. Based on our findings, we propose that rib pneumaticity among apatosaurine sauropods (and potentially all diplodocids) is individually variable, in addition to being a function of ontogeny. More broadly, we conclude that rib pneumaticity among diplodocids is morphologically variable when present and individually expressed rather than being ubiquitous throughout the clade. Our findings are consistent with the hypothesis that pneumatic ribs evolved independently between Diplodocidae and Titanosauriformes and make for poor clade-level characters among diplodocids.