Fossil sirenian specimens collected in 1964 by the late R. J. G. Savage's expeditions in north-central Libya are described. They come from early middle Eocene (lower Lutetian, 47.8–43.6 Ma) deposits at the locality of Bu el Haderait and represent a new genus and species, Libysiren sickenbergi. This animal is the largest known protosirenid, and the largest Eocene sirenian known to date (condylobasal length >420 mm). Its dental formula was apparently 184.108.40.206, with five premolar loci as in all other Eocene sirenians, but the teeth are mostly not preserved. Its postcranial skeleton is unknown except for the atlas, a thoracic vertebra, and rib fragments. Stable isotopes indicate a mostly seagrass diet and a habitat of fully marine salinity. The Protosirenidae presently comprise the genera Protosiren, Ashokia, and Libysiren, with their interrelationships unresolved. Together, they are most parsimoniously regarded as a paraphyletic group basal to both Trichechidae and Dugongidae. However, as more of their morphology and diversity are revealed, they may prove to be more closely allied to the former and may shed crucial light on the still-mysterious origins of the trichechids (manatees).