A pitfall trap is a trapping pit for small animals, such as insects, amphibians and reptiles. Pitfall traps are a sampling technique, mainly used for ecology studies and ecologic pest control. Animals that enter a pitfall trap are unable to escape. This is a form of passive collection, as opposed to active collection where the collector catches each animal (by hand or with a device such as a butterfly net).

Pitfall traps come in a variety of sizes and designs. They come in two main forms: dry and wet pitfall traps. Dry pitfall traps consist of a container (tin, jar or drum) buried in the ground with its rim at surface level used to trap mobile animals that fall into it.

Wet pitfall traps are basically the same, but contain a solution designed to kill and preserve the trapped animals. The fluids that can be used in these traps include formalin (10% formaldehyde), ethanol, ethylene glycol, propylene glycol, acetic acid, chloral hydrate, or even (with daily checked traps) plain water. The choice of preservative is dependent on the study, since different preservatives evaporate at different rates, and are other considerations such as which preservative is better or worse suited to preserve DNA or morphological information. A little detergent is usually added to break the surface tension of the liquid to promote quick drowning.

The opening is sometimes covered by a sloped stone or lid or some other object. This is done to reduce the amount of rain and debris entering the trap, and to prevent animals in dry traps from drowning (when it rains) or overheating (during the day) as well as to keep out predators. Some more complicated designs also include a vertebrate exclusion fence between the rain-guard and the edge of the trap to prevent bycatch of vertebrates such as lizards.

Traps may also be baited. Lures or baits of varying specificity can be used to increase the capture rate of a certain target species or group by placing them in, above or near the trap. Examples of baits include meat, dung, fruit and pheromones.

Some designs include fence-lines sometimes called guidance barriers or drift-fences which are small fences that funnel the target organisms towards the trap, resulting in a significantly higher species richness compared with designs without.

Disturbing the soil in an area can have an effect on the activity of ground dwelling arthropods in an area, so sometimes a rim-plate is attached to the trap container which allows the trap to be swapped out easier and avoids the effects of disturbing the soil.

Pitfall traps can be used for various purposes: