Ernst Vilhelm Østrup (21 September 1845, in Roskilde – 16 April 1917, in Frederiksberg) was a Danish botanist and phycologist, mainly working on diatoms.

In 1873 he received his cand. polyt. degree, and later worked as a schoolteacher in Copenhagen. His diatom research largely dealt with species found in Denmark, Greenland, Iceland, Faeroe Islands and Jan Mayen.^[1]

The diatom genus Oestrupia (Heiden)^[2] and the species Diploneis oestrupii (Hust.), Surirella oestrupii (Gran) and Navicula oestrupii (Cleve) are all named in his honour.

• Ferskvands-Diatoméer fra Øst-Grønland. Meddelelser om Grønland 15: 251–290. 1898.
• Beiträge zur Kenntnis der Diatomeenflora des Kossogolbeckens in der nordwestlichen Mongolei. Hedwigia 48 (1-2): 74–100. 1909.
• Danske Diatoméer. C.A. Reitzel, København. 323 pp. (1910)
• Diatoms from North-east Greenland (N of 76 N.Lat.) collected by the Denmark Expedition. Meddelelser om Grønland 43 (10): 193–256. 1910.
• Diatomaceæ ex insulis Danicis Indiæ Occidentalis imprimis a F. Børgesen lectæ. Dansk Botanisk Arkiv vol. 1 (1): 1-29. (1913)
• Marine diatoms from the coasts of Iceland Archived 2008-03-07 at the Wayback Machine. The Botany of Iceland, edited by L. Kolderup Rosenvinge & E. Warming, J. Frimodt, Copenhagen, and John Wheldon and Co., London; Vol. 1, Part 2, pp. 345–394. 1916.
• Fresh-water diatoms from Iceland. The Botany of Iceland, edited by L. Kolderup Rosenvinge & E. Warming, J. Frimodt, Copenhagen, and John Wheldon and Co., London; Vol. 2, Part 1, pp. 1–100. 1918.