An award-winning police sergeant in Missouri has filed a discrimination lawsuit against his employer, alleging that he was told he would not be promoted unless he toned down his “gayness.”

The sergeant in the St. Louis County Police Department, Keith Wildhaber, said that over the years his performance reviews have said he “exceeds standards” or is “superior” in all categories. But in 2014, the suit says, after Sgt. Wildhaber became one of nine top candidates eligible to move up to lieutenant, he and another officer, who had disciplinary and performance issues, were the only ones denied promotion.

That same year, the lawsuit says, the sergeant was told by John Saracino, a member of the St. Louis County Board of Police Commissioners, which supervises the department, that the command staff “has a problem with your sexuality.”

“If you ever want to see a white shirt (i.e., get a promotion), you should tone down your gayness,” Mr. Saracino said, according to the suit.