The Goeldi's marmoset or Goeldi's monkey (Callimico goeldii) is a small, South American New World monkey that lives in the upper Amazon basin region of Bolivia, Brazil, Colombia, and Peru. It is the only species classified in the genus Callimico, and the monkeys are sometimes referred to as "callimicos". The species takes its name from its discoverer, Swiss-Brazilian naturalist Emil August Goeldi.

Goeldi's marmosets are blackish or blackish-brown in color and the hair on their head and tail sometimes has red, white, or silverly brown highlights. Their bodies are about 8–9 inches (20–23 cm) long, and their tails are about 10–12 inches (25–30 cm) long. They weigh about 0.4835 Kg in captivity and 0.500 Kg in the wild. Their digits have claw like nails except for the hallux, which serve for clinging, scansorial travel, and to extract food from trees.