- Ulula
- Surnia ulula [Kupon 855] [Coupon 855]
- Uncia
- Panthera uncia [Známka 894] [Stamp 894]
- Undulatus
- Melopsittacus undulatus [Známka 409] [Stamp 409]
- Ungulatus
- Unicornis
- Rhinoceros unicornis [Známka 939] [Stamp 939]
- Unicum
- Dendrobium unicum [Kupon 730] [Coupon 730]
- Unio
- Unio pictorum [Aršik 563] [Souvenir Sheet 563]
- Universitas
- Universitas Comeniana Bratislavensis condita MCMXIX [Známka 817] [Stamp 817]
- Upupa
- Upupa epops L. [Známka 210] [Stamp 210]
- Upupa epops [Známka 648] [Stamp 648]
- Uralensis
- Urbium
- Praga mater urbium [Známka 1586] [Stamp 1586]
- Urogallus
- Ursus
- Ursus arctos [Známka 955] [Stamp 955]
- Ursus archtos [Známka 1572] [Stamp 1572]
- Ursus arctos [Známka 816] [Stamp 816]
- Ursus maritimus [Známka 938] [Stamp 938]