Service employes at the Washington Hospital Center have rejected a new three-year contract reached in negotiations this week between hospital officials and the Service Employees Union Local 722.

Local president Marchel Smiley said his members rejected the pact, which called for a 25 percent raise in wages, because it did not contain adequate job protection and seniority clauses. Union members are worried about their jobs, he said, because President Reagan has promised to cut several federal programs that affect hospitals.

The union represents 1,400 employes at the hospital, including licensed practical nurses, technicians and clerks. Smiley said the members voted 414 to 268 to reject the contract.

Allen G. Siegel, chief negotiator for the hospital, said union members' fears are unfounded. He called the contract one of the best in the hospital's history.

Federal mediators yesterday said both sides have agreed to another round of talks Tuesday even though the union's current contract expires at midnight Monday. Federal law requires hospital unions to submit to arbitration and then give hospitals a minimum 10-day warning before calling a strike.

This is the third contract that the union, which was formed in 1975, has negotiated with the hospital. The previous contracts were reached without a strike, Smiley said.