An advertising agency or ad agency is a service business dedicated to creating, planning and handling advertising (and sometimes other forms of promotion) for its clients. An ad agency is independent from the client and provides an outside point of view to the effort of selling the client's products or services. An agency can also handle overall marketing and branding strategies and sales promotions for its clients.

Typical ad agency clients include businesses and corporations, non-profit organizations and government agencies. Agencies may be hired to produce an advertising campaign.