Always include "the" when describing divisions and departments of companies and schools, so "He works in the sales department" is correct. Other examples: "The Chemistry department at my college," "The state health department," and "the marketing division of my company."

Thank you very much Nancy and Xarmanla :)

He works as a salesman. He works in sales. He works in retail sales. When you use the name of the department, then you need the definite article.

Yes, as Dorita said, definitely use "the". He works in the sales department.