Ryu Ho-jeong, 28, and a member of the minor progressive Justice Party, faced intense scrutiny in various online communities this week which including misogynistic and sexist comments likening her to a "call girl" or a "bar hostess."
An outpouring of negative comments clashed with cheers online supporting her freedom of choice -- an active debate, analysts say, reflecting a deep generational and cultural divide in a largely conservative society over social norms.
"The job of an assemblyman is public service which all citizens are keeping [an] eye on, so they should stick to formality when carrying out their job," Jung Yoojin, a college senior studying in South Korea, told ABC News. "Even college students dress up formally when they make presentations in class. Showing up in a red dress seems like an action that goes against the national sentiment."
But Ryu, who is the youngest member of the chamber, shrugged off the negative reactions saying that "the authority of the National Assembly is not built upon wearing a suit."
"The National Assembly is centered on middle-aged men in their 50s, and I wanted to break this practice represented by dark suits and ties by wearing casual clothes," she told ABC News.
South Korea's 300-member National Assembly has become diverse in age and sex in the past decades but still predominantly comprises middle-aged men. Ninety-seven percent of lawmakers are over 40, while only 3% are under 30, and 84.3% are male, according to the National Election Commission's report.
"Korean people tend to regard the National Assembly as a serious and solemn place of power. There are also remnants of formality that still dominate this society," Lee Joohee, director of Ewha Woman's University Department of Sociology Career Development Center told ABC News.
"I see Ryu's attire as a representation of the people in their 20s," Kim Sunwoo, a job-seeking college graduate, said. "Ryu knows her supporters and knows how to consolidate those people by representing them at the National Assembly."
With young voters in mind, fellow lawmakers were quick to support Ryu's unconventional outfits to work which in the past included jeans, shorts, a yellow purse and a backpack.
"I cannot agree with the excessive criticism of what she [Ryu] wore. I appreciate her for breaking the excessive rigorism and authoritarianism inside the National Assembly," Assemblywoman Ko Min Jeong posted on her Facebook page Thursday.
"To me, the National Assembly is a workplace. I wear what I believe is comfortable to work in," Ryu said. Ryu was surprised by how many people took interest in her casual attire to the National Assembly, as she had been doing so since the assembly opened two months ago. The red dress was a common dress that would have been worn by any other business woman her age, and that was why she chose to wear it to work, she said. The red patterned dress by Jucy Judy, a Korean retail brand, has now sold out.
ABC News' Hakyung Kate Lee, HyunJoo Haley Yang and Aaron Kwon contributed to this report.