Mercedes-Benz of North America, best known for its luxury cars, will open a truck assembly plant in Hampton, Va., making it the first foreign manufacturer with such a plant in the United States, the company announced yesterday.

The $6.6 million plant, located minutes from the port at Newport News, is scheduled for construction this spring, and the first trucks should be wheeled off the assembly lines in the spring of 1980, the company announcement said.

The company expects to hire about 160 workers in the beginning, mostly from the Hampton area,d with an annual payroll of about $3 million. The Hampton and Newport News area has the highest unemployment rate, 6 percent, of Virginia's 16 major labor markets.

The company, a subsidiary of Daimler-Benz AG of West Germany, has signed an option to buy 50 acres in the Copeland Industrial Park for the plant and has taken an additional option on another 50 acres for possible future development, the announcement said.

The plant will roll off 6,000 diesel trucks annually with output during its first year at 4,000 trucks, the announcement said. Mercedes-Benz began its truck marketing program in this country nine years ago, the company siad, and now markets a line of medium to large diesel trucks ranging in price from $16,369 to $26,086, according to a company spokesman, Mike Geylin.

The world's oldest automobile manufacturer, the company chose Hampton because it's "a very desirable site for shipping and the port makes it accessible to ring parts in" from its operations in Brazil, Geylin said. A unique feature of the Hampton plant, the company said, will be that sub assemblies shipped from Brazil rather than small pieces will be joined at the plant.

Company officials in New York cited the decreasing value of the dollar and increasing interest in the U.S. market as reasons for the move.

Geylin also sia the city's labor market and taxes were attractive and by assembling the vehicels in this country it can avoid paying the 25 per cent tariff on imported trucks. That tariff compares to 3 percent levied on imported automobiles, Geylin siad.

Last year the company sole 2,251 trucks in the United States compared to 1,800 the year before. The company predicts the sale of 3,500 trucks this year in the United States, Geylin said.

The German company expects to fare better than its Swedish competitor, Volvo, which planned to open an auto assembly plant in enighboring Chesapeake, Va., three years ago but has virtually abandoned its $25 million plant because of declining sales in the United States.