“I’m not running, but I’m going to keep on working and speaking and standing up for what I believe,” Mrs Clinton told New York’s News 12 TV channel.
As the Democratic candidate in 2016, Mrs Clinton was widely expected to become the first female US president, before a shock defeat by Mr Trump.
Asked by News 12 if she would run again for any public office in future, she said: “I don’t think so.”
The interview is the first time Mrs Clinton has definitively rebutted speculation that she might take on Mr Trump again in 2020.
She said she had spoken to some of the declared 2020 Democratic candidates. “I’ve told every one of them, don’t take anything for granted,” she said.
Mrs Clinton was the first female presidential nominee for a major US party in the nation’s history. Her ascension to the top office was widely seen as all but guaranteed, but she was dogged by accusations of being overly reliant on wealthy donors, too close to Wall Street and out of touch with younger voters.
She was relentlessly pilloried by Mr Trump, who goaded crowds on the campaign trail to chant “Lock her up”.
Her defeat prompted a period of soul-searching for the Democratic Party, which is now gearing up for a wide-open primary contest that has already seen more than 10 contenders announce a run.