Otocinclus arnoldi is a species of armoured catfish native to South America, where it occurs in the lower río Paraná drainage, in the lower and middle rio Uruguai and in the río de La Plata. This species reaches a maximum length of 4.8 centimetres (1.9 in) (SL).

Named in honor of German aquarist Johann Paul Arnold (1869–1952), who "presented" type specimen to the British Museum.

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