"As one who celebrates and strives for diversity, individual freedom and respect for all lifestyles, I will not participate in dressing or associating in any way with the next first lady," she wrote in an open letter published Thursday.
"The rhetoric of racism, sexism and xenophobia unleashed by her husband's presidential campaign are incompatible with the shared values we live by."
"I am well aware it is not wise to get involved in politics," Theallet added. "That said, as a family-owned company, our bottom line is not just about money."
The letter has prompted a flood of reaction on social media, much of it negative. Theallet, 52, who has worked in the United States for more than fifteen years, is a regular fixture at New York's fashion week.
Current US First Lady Michelle Obama, has been dressed by Theallet several times and Michelle has provided welcome publicity by wearing several of the designer's dresses since 2009.
"She has contributed to having our name recognized and respected worldwide," Theallet wrote of the current first lady. "Her values, actions and grace have always resonated deeply within me."