In the transition from a terrestrial to an aquatic environment, sirenian marine mammals reduced and lost their hindlimbs and developed a horizontal tail, the main propulsive organ in extant sirenians. Quadrupedal forms are only known from the Eocene and are represented by three different taxa: the amphibious prorastomids, the aquatic quadrupedal protosirenids and Sobrarbesiren cardieli, a four-legged sirenian from the middle Eocene of Spain, considered the sister taxon of the aquatic Dugongidae. This ecological shift was naturally associated with adaptations, among others, of the skeleton. However, sirenian hindlimb bones have been poorly studied because of the scarce material available in the fossil record. Here we describe in detail the hindlimb bones of Sobrarbesiren, analyzing their functional morphology and comparing them with other basal sirenians and cetaceans. Sobrarbesiren had strong control of its hindlimbs, which were capable of a great variety of movements. Based on the presence of a strong sacroiliac articulation, we propose that it swam by dorsoventral pelvic undulation combined with pelvic paddling analogous to some protocetid archaeocete whales. We also conducted the first microanatomical analysis of hindlimb bones of an Eocene sirenian. Data reveal extreme inner compactness in the Sobrarbesiren innominate bone and femur, with the first description of 2 osteosclerosis in an amniote innominate bone combined with the highest degree of osteosclerosis observed in amniote femora. The results confirm that the microanatomical changes precede the external morphological changes in such ecological transitions. The process of adaptation of sirenians to an aquatic life was thus a more complex process than previously thought.