Gamers take on the role of the gifted hacker Aiden Pearce as he uses his skills to manipulate an open-world version of Chicago in Watch Dogs. A giant network of computers known as the Central Operating System (CtOS) connects and monitors the entire city, from traffic lights and surveillance cameras, to the cell phones and personal computers of private citizens. As a younger man Aiden's affiliation with the criminal underworld led to his niece's death, so when he learns his family may be in danger again, he uses his expertise to hack into CtOS and hand out vigilante justice.

Watch Dogs lets players explore Chicago in a variety of ways, with Aiden walking the streets, entering buildings, using mass transit, and driving more than 65 different vehicles. Aiden has access to the CtOS from his smart phone, meaning an average walk can include him altering traffic lights to cause accidents, tapping into surveillance cameras, or getting detailed personal information on pedestrians, including access to their bank accounts. Players are free to use this power in any way they wish, creating havoc in the streets and leading police on fruitless high-speed chases, or stealing from the rich and using a computer to fight criminal activity from miles away. While using stealth is one of Aiden's specialties, he's also quite adept with a baton, and gamers can wield more than 30 different firearms.

The single-player campaign in Watch Dogs is structured in much the same way that Ubisoft's open-world franchises Assassin's Creed and Far Cry are, with players heading to a central location in each of the city's districts in order to unlock a variety of side missions, mini-games, and collectible items. The game also includes several multiplayer modes, including an eight-player free roam mode, car and boat races, and an asynchronous mode in which gamers attempt to hack one another without being detected.