| Unionopterus Temporal range: Tournaisian-Viséan,
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| Type and only known specimen of U. anastasiae | |
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| Restoration of the carapace and first body segment of U. anastasiae | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Order: | †Eurypterida |
| Superfamily: | †Adelophthalmoidea (?) |
| Family: | †Adelophthalmidae (?) |
| Genus: | †Unionopterus Chernyshev, 1948 |
| Type species | |
| †Unionopterus anastasiae Chernyshev, 1948
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Unionopterus is a genus of eurypterid, an extinct group of aquatic arthropods commonly known as "sea scorpions". Fossils have been registered from the Early Carboniferous period. The genus contains only one species, U. anastasiae, recovered from deposits of Tournaisian to Viséan stages in Kazakhstan. Known from one single specimen (now presumed to be lost) which was described in a publication of Russian language with poor illustrations, Unionopterus' affinities are extremely poorly known.
Unionopterus was described in 1948 after the discovery of its only known specimen in the Karaganda Formation of Kazakhstan (at that time part of the Soviet Union). This was one of the first discoveries of eurypterids in the Soviet Union and Carboniferous eurypterids in general. It was probably a swimming organism as well as other eurypterids, although not as good as its relatives.
There are numerous factors that have made Unionopterus a problematic genus for eurypterid researchers. It has been placed in the family Adelophthalmidae and it has even been speculated that a species of Adelophthalmus, A. dumonti, actually belongs to Unionopterus, but this cannot be confirmed. Many authors have chosen to completely ignore the genus during phylogenetic studies, making Unionopterus an enigmatic eurypterid.