A police dog is a dog that is specifically trained to assist police and other law-enforcement personnel. Their duties include searching for drugs and explosives, locating missing people, finding crime scene evidence, protecting people, and attacking criminals who are fleeing from or attacking police officers. The most commonly used breeds are the German Shepherd, Belgian Malinois, Bloodhound, Dutch Shepherd, and the retriever breeds Recently, the Belgian Malinois has become the dog of choice for police and military work due to their intense drive and focus. Malinois are smaller and more agile than German Shepherd Dogs, and have fewer health issues. However, a well-bred working line German Shepherd Dog is just as successful and robust as a Malinois. Police dogs must remember several verbal cues and hand gestures.

In many countries, the intentional injuring or killing of a police dog is a criminal offense.

In some English-speaking countries, police dog units are often referred to as K-9 or K9, which is a homophone upon the word.