The New York Daily News and the New York Post reported Friday that Nixon has been dating a woman since January. The woman is neither in the public eye nor living with Nixon.
"We will not confirm or deny anything," Nixon's publicist, Carrie Ross, told AP.
"My private life is private," Nixon told the Daily News. "But at the same time, I have nothing to hide. So what I will say is that I am very happy."
The actress split in June 2003 with photographer and high school sweetheart Danny Mozes, whom she'd been with for 15 years (but never married). They now share custody of their two children.
If "Sex and the City" were to be believed, the revelation would have been more expected from Kim Cattrall's character, Samantha, whose many, many TV conquests included one woman.
Nixon's lawyer character was the cool professional of the bunch, although her boss mistook her for a lesbian in the first season and set her up with a woman, resulting in a single kiss. But Miranda never dallied further and ended up married to her bartender boyfriend.
After two previous nominations, Nixon, 38, won an Emmy this year for her performance on the HBO show. She is working on "Tanner on Tanner," a Robert Altman and Gary Trudeau miniseries for the Sundance Channel.