Kelly Services, Inc. (formerly Russell Kelly Office Service and Kelly Girl Service, Inc.) is an American office staffing and workforce solutions company that operates globally. The company places employees at all levels in various sectors including the financial services, information technology, and law industries. It also offers professional services, including human resource and management consulting, outsourcing, recruitment, career transition, and vendor management services. Kelly Services was founded by William Russell Kelly in 1946 and is headquartered in Troy, Michigan. In 2015, the company reported 8,100 employees, $5.5 billion in revenue, and placed 550,000 employees to work in positions in various sectors, making it one of the world's largest staffing firms.

William Russell Kelly founded Russell Kelly Office Service in 1946, initially providing office services performed at Kelly's offices, rather than providing temporary staffing at the client's location. However, customers asked for the work to be performed at their locations, and the temporary staffing concept was born out of responding to this type of request. It was in 1946 that the first 'Kelly Girls' came into scope and the first 'Kelly Girl' was Adelaide Hess Moran who was able to demonstrate that this new working sphere for women could be one to enjoy.

The temporary workers, who were usually female, became known as 'Kelly girls'. In recognition of this, the name of the company was changed to Kelly Girl Service, Inc. in 1957. The term 'Kelly girl' became a widely used term for a temporary worker, regardless of the worker's company affiliation or gender. In 1966 the name of the company was changed to Kelly Services, Inc., in order to reflect an expanding range of services.

The current Chairman of the Board of Directors is Terence E. Adderley, and the CEO is Carl T. Camden.