The golden otocinclus (Macrotocinclus affinis, formerly Otocinclus affinis) is one of the smallest known suckermouth catfish, often called a 'dwarf oto'. Endemic to Southeast Brazil, this herbivorous, rheophilic, bottom-feeder only grows to around 4 cm (1.6 in) in length. The close relatives of this small fish are often used for the purpose of controlling algae in small home aquariums, under the name Otocinclus affinis. In reality, they belong to the species O. vittatus, O. vestitus and O. macrospilus and O. huaorani. The real M. affinis is not present in the aquarium hobby.

M. affinis is a Batesian mimic of Corydoras nattereri. Due to its narrow stripe, M. affinis resembles this Corydoras species more than it does Otocinclus species.

M. affinis prefers an aquarium with fast moving water, sandy substrate, and plenty of hiding areas (old broken clay pipes is a preferred hiding habitat). Preferred pH is slightly acidic with a tank temperature of 21–27 °C (70–81 °F). This particular species is not as hardy as many of the other Otocinclus species.