LOS ANGELES — Hollywood’s two biggest public relations agencies announced a merger on Tuesday, creating a combined firm with more than 500 clients as competition from boutique publicity agencies intensifies.

Both of the merging agencies, PMK-BNC and Rogers & Cowan, are owned by Interpublic Group, but they have competed as stand-alone public relations and branding agencies. By joining forces they expect to reduce costs while providing clients with greater resources.

“It does create efficiencies, but it’s also about servicing our clients better,” Alan Nierob, a co-president of Rogers & Cowan, said in a telephone interview.

The new agency, whose name has not yet been determined, will be run by Cindi Berger as chairman and Mark Owens as chief executive. Ms. Berger previously led PMK-BNC; Mr. Owens was her counterpart at Rogers & Cowan. Mr. Nierob, as chairman of the entertainment division, will oversee celebrity clients like Denzel Washington, Brie Larson, Robert Redford, Michael B. Jordan, Felicity Jones and Chris Pratt.