A Denobulan lullaby was a lullaby originating with the Denobulans.
In 2154, Doctor Phlox offered to sing a Denobulan lullaby to Porthos during his attempt to lift Porthos' mood, since its owner, Jonathan Archer, was missing. Phlox then recalled that his singing would often drive his children to tears, and chose not to sing such a lullaby to Porthos. (ENT: "Storm Front")