The Tosk are a species who live in the Gamma Quadrant and are part of the Dominion. Tosk are bred and trained, from birth, to take part in a grand adventure they refer to simply as the "hunt": At a predetermined time, a Tosk will set out into the galaxy while being relentlessly pursued by Hunters. Tosk view the hunt not as a burden, but rather as the greatest adventure that anyone could ever desire. Tosk will do everything possible to avoid capture; if captured alive, they face a grim fate - to be taken back to the home world and put on public display where others will mercilessly ridicule them. Tosk are forbidden from discussing themselves or their role in the hunt with outsiders. After a Tosk came to Deep Space Nine requesting help repairing its spaceship, it was briefly captured, but CPO Miles O'Brien allowed the Tosk to escape, whereupon the Hunters eagerly resumed their pursuit. Subsequently, DS9, and the rest of the Alpha Quadrant, were declared off-limits for future hunts.

Tosk are remarkably self-sufficient, requiring little sleep and no food (nutrient fibers stored throughout their bodies provide all nourishment they need). They share some physical features with Jem'Hadar (and, to a lesser extent, with the pursuing Hunters) - a popular theory is that all are descended from the same original species. Tosk also share the Jem'Hadar ability to cloak themselves, becoming completely invisible, a trait which proves very valuable in the hunt.

Tosk are regarded by their Hunters with respect and care, and are viewed by the Hunters as the personification of everything that is noble and courageous. Like Klingons, all Tosk aspire to ultimately die with honor.