A detection dog or sniffer dog is a dog that is trained to use its senses to detect substances such as explosives, illegal drugs, wildlife scat, semen, currency, blood, and contraband electronics such as illicit mobile phones. The sense most used by detection dogs is smell. Hunting dogs that search for game, and search and rescue dogs that work to find missing humans are generally not considered detection dogs but fit instead under their own categories. There is some overlap, as in the case of cadaver dogs, trained to search for human remains.

A police dog is essentially a detection dog that is used as a resource, for police in specific scenarios such as conducting drug raids, finding missing criminals, and locating stashed currency. Frequently, detection dogs are thought to be used for law enforcement purposes. Experts say that dog-sniff evidence should not be used in the criminal justice system, pointing to wrongful convictions, human biases that skew animal behavior, and the lack of systematic research into what dogs detect or how they do it.

Although detection dogs are often used for law enforcement purposes, they are also used as a valuable research tool for wildlife biologists. In California, detection dogs are trained to discover quagga mussels on boats at public boat ramps because they are a harmful invasive species for the environment. Detection dogs also tend to be employed for the purposes of finding and collecting the feces of a diverse array of species, including caribou, black-footed ferret, killer whale, and Oregon spotted frog. This process is known as wildlife scat detection.

Detection dogs have been trained to search for many things, both animate and inanimate, including:

Sniffer dogs can be trained to locate small infestations of invasive and non-native weeds. The world's first Spotted Knapweed K-9 detection program successfully completed field-testing for Montana State University in 2004. Upon completion of the testing, Knapweed Nightmare was finding low densities of non-native invasive knapweed rosettes in the field with a 93% overall success rate. She followed it up with 98% in the final trials in open fields, demonstrating that dogs can effectively detect low densities of invasive plants.

Detection dogs are able to discern individual scents even when the scents are combined or masked by other odors. In 2002, a detection dog foiled a woman's attempt to smuggle marijuana into an Australian prison in Brisbane. The marijuana had been inserted into a balloon, which was smeared with coffee, pepper, and petroleum jelly and then placed in her bra.

Detection dogs are often specially trained by handlers to identify the scent of bed bugs. With the increased focus on green pest management and integrated pest management, as well as the increase in global travel and shared living accommodations, bed bugs have become more prevalent. Detecting bed bugs is a complicated process because insects have the ability to hide almost anywhere. Detection dogs help solve this problem because of their size, speed, and sense of smell. Detection dogs use their unique ability to smell in parts per trillion in order to track bed bugs in every phase of their life cycle. They can find bugs in places humans cannot such as wall voids, crevices, and furniture gaps. Dogs are also a safer alternative to pesticide use. If detection dogs can find out exactly where bed bugs are located, they can minimize the area that needs to be sprayed.

The National Pest Management Association released their "Bed Bug Best Management Practices" in 2011 which outlines the minimum recommendations regarding not only treatment, but the certification and use of bed bug detection canines. The NPMA's Best Management Practices emphasizes the importance of having bed bug detection dog teams certified by third party organizations who are not affiliated to the trainer or company that sold the canine.