Diunatans is considered to be a stem balaenopterid because it falls outside the Balaenoptera+Megaptera clade, which includes all living rorquals.

The type species is D. luctoretemergo, named after the motto of Zeeland, "Luctor et Emergo" (Latin for "I struggle and I emerge").

Several distinguishing characteristics can be seen in the skull of Diunatans, including a large occipital condyle and very small nasal bones compared to other rorquals.

English paleontologist Richard Owen named Balaenoptera definata, B. emarginata, and B. gibbosa in 1844.

In the late 19th century, Belgian paleontologist Pierre-Joseph van Beneden named B. borealina, B. musculoides, B. rostratella, B. sibbaldina, Plesiocetus goropii, Megapteropsis robusta, and Megaptera affinus.