In this study plesiosaur limb bones of four specimens from the Late Jurassic of Svalbard have been studied to map gross internal structure and microstructure and compare this to extant marine reptiles and mammals. Two specimens, one juvenile (species A) and one subadult (species B) are from the Adventdalen Group, Janusfjellet subgroup, Agardhfjellet formation, Slottsmøya member, dated as mid-Volgian; one adult (species D) from Agardhfjellet, also dated mid-Volgian; and one juvenile (species C) of unknown age. The bones examined are propodials, phalanges, mesopodials and metapodials. This study is the first to describe the microstructure of the latter two. The inner bone structure fits an active marine animal living in cold water. Many of the features in the present material are often found in animals with rapid growth and high metabolism, including secondary osteons, high vascularization, pits on the outside of the epiphysis, woven and possibly fibro-lamellar bone. The long bones have two endochondral cones in a periosteal sheath, with a small medullary cavity. The propodials have a defined and quite compact cortex in the subadult, a finding that rejects the view that all plesiosaur bones became more porous through ontogeny. There is a microstructural difference between bones from different ontogenetic stages: juvenile bones have no remodeling and not completely ossified endochondral cones. Three bones have regular trabecular rings in a porous cortex, maybe resulting from cyclic growth caused by seasonality, migration or ontogeny.