Cyrtulus undatus, common name Legrand's wavy spindle, is a species of sea snail, a marine gastropod mollusk in the family Fasciolariidae, the spindle snails, the tulip snails and their allies.
The species was originally described as Murex undatus by Johann Friedrich Gmelin in 1791.
The shell is large and fusiform, reaching lengths of about . It is ventricose with a high, pointed spire. The whorls show undulating angular sculpture and fine transverse striations. The base of the whorls may show nodose ornamentation. The aperture is elongate and the outer lip is denticulate.
The species was originally described as Murex undatus by Gmelin in 1791. It was later placed in the genera Fusus and Fusinus. A revision of the subfamily Fusininae by Vermeij and Snyder (2018) transferred the species to the genus Cyrtulus.
Cyrtulus undatus occurs in the Indo-West Pacific, including the Philippines, Japan (Wakayama), Indonesia, Vietnam, the Solomon Islands, the Marshall Islands and Tahiti.
The species inhabits sandy or muddy bottoms, typically at depths greater than 20 m. Living individuals are occasionally observed on sand at night. Empty shells are frequently occupied by hermit crabs.