A "professional employer organization," as further defined in RCW 50.04.298(1), is a person or entity that enters into an agreement with one or more client employers to provide professional employer services in a coemployment relationship. The professional employer services may include functions such as human resources, risk management, payroll administration services, or unemployment insurance. Both the professional employer organization and the client employer are considered coemployers.

A "professional employer organization" includes entities that use the term "staff leasing company," "permanent leasing company," "registered staff leasing company," "employee leasing company," or "administrative employer" and provide professional employer services to client employers. It does not include independent contractors under RCW 50.04.140, temporary staffing services companies and services referral agencies under RCW 50.04.245, third-party payers under RCW 50.04.248, labor organizations, or common paymasters or common pay agents under RCW 50.04.065.