Otradnocetus was described by the Georgian paleontologist Guram Mchedlidze in 1984 on the basis of GNM CO 1–90, a partial skeleton including the incomplete skull, mandible, 43 vertebrae and both forelimbs (humeri, radii, ulnae, a few carpals, metacarpals, and phalanges) with scapulae. The type specimen was found in Otradnaya, in the northwestern Russian Caucasus Mchedlidze assigned Otradnocetus to the family Cetotheriidae, at the time a wastebasket for baleen-bearing mysticetes that did belong to any crown-group mysticete group (followed by McKenna and Bell 1997).^[1][2] Later recognition of Cetotheriidae sensu McKenna and Bell (1997) as paraphyletic/polyphyletic rendered the systematic position of Otradnocetus, and the cladistic analyses of Gol'din & Steeman (2015) and Gol'din (2018) recovered Otradnocetus as sister to Parietobalaena securis in a clade basal to Cetotheriidae and a clade formed by Pinocetus, Cephalotropis, and Neobalaenidae. The juvenile specimen VSEGEI 2401, referred to Cetotherium aff. mayeri by Riabinin (1934)^[3] and also found in the Middle Miocene of the Russian Caucasus, was referred to Otradnocetus by Gol'din (2018).^[4][5]

Otradnocetus is a medium-size cetothere 4-5 meters in length. It differs from other members of Cetotherioidea and is most similar to Parietobalaena in having a very short ascending process of the maxilla, a short lateral process of the maxilla, an anterior end of nasal located anterior to the rostrum base, and a supraorbital process of the frontal bone directed perpendicular to the anteroposterior axis of the skull. Differences from Parietobalaena include a supraorbital process of the frontal bone not elongated at the lateral end and a robust medially bent coronoid process of the mandible.^[5]