The New York Daily News and the New York Post reported Friday that the 38-year-old actress has been dating a woman for 10 months now.
"My private life is private. But at the same time, I have nothing to hide. So what I will say is that I am very happy," Nixon told the New York Daily News Friday when asked whether she is involved with another woman.
Meanwhile, Nixon's publicist Carrie Ross told AP: "We will not confirm or deny anything."
In June 2003, Nixon split with her boyfriend of 15 years, photographer and high school sweetheart Danny Mozes. The two share custody of their two children and are said to be on friendly terms, according to the Daily News.
The paper reports the woman is neither in the public eye nor living with Nixon, who has managed to keep the new romance under wraps by going to media events alone or with her mother.
After two previous nominations, Nixon won an Emmy this year for her performance on the HBO show. She is working on Tanner on Tanner, a Robert Altman and Garry Trudeau miniseries for the Sundance Channel.