SEOUL, South Korea — Jasmine Lee realizes just how Korean she has become when she breaks out in the language, forgetting that her Filipino mother on the other end of the phone can’t understand her. But she is reminded of the limits of assimilation when Koreans, impressed by her fluency, comment, “You sound more Korean than Koreans do.”

Ms. Lee, 35, who was born Jasmine Bacurnay in the Philippines, made history in April when she became the first naturalized citizen — and the first nonethnic Korean — to win a seat in South Korea’s National Assembly. Her election reflected one of the most significant demographic shifts in the country’s modern history, a change Ms. Lee says “Koreans understand with their brain, but have yet to embrace with their heart.”

Only a decade ago, school textbooks still urged South Koreans to take pride in being of “one blood” and ethnically homogeneous. Now, the country is facing the prospect of becoming a multiethnic society. While the foreign-born population is still small compared with that of countries with a tradition of immigration, it is enough to challenge how South Koreans see themselves.

“It’s time to redefine a Korean,” said Kim Yi-seon, chief researcher on multiculturalism at the government-financed Korean Women’s Development Institute. “Traditionally, a Korean meant someone born to Korean parents in Korea, who speaks Korean and has Korean looks and nationality. People don’t think someone is a Korean just because he has a Korean citizenship.”