SEOUL, South Korea — At a time of growing unease in the alliance between the United States and South Korea, a hairy diplomatic issue has surfaced: the American ambassador’s mustache, which has become an object of ridicule and resentment among many South Koreans.

On Thursday, the envoy, Harry B. Harris Jr. — a retired Navy admiral who was born in Japan to a Japanese mother and an American Navy officer — defended his mustache in the face of some sentiment that it was a reminder of Japan’s brutal colonial rule over South Korea.

South Koreans hold a long-running animosity toward Japan​ because of that period, and many recall that Japanese governors-general ​who ruled Korea​ from 1910 to 1945 wore mustaches​.

“My mustache, for some reason, has become a point of some fascination here,” Mr. Harris, 63, told foreign news reporters in Seoul on Thursday. “I have been criticized in the media here, especially in social media, because of my ethnic background, because I am a Japanese-American.”