Fifty-two percent of rich women came into their marriage or live-in relationship with equal or more assets than their spouse or partner, according to a study released by the Bank of America.
Respondents had at least US$3 million in investable assets.
"Women are both wealth creators and wealth inheritors," Chris Heilmann, chief fiduciary executive at US Trust, said.
"They are bringing assets to a relationship."
Women are contributing assets from their careers, inheritances and divorce settlements, Heilmann said.
Thirty per cent of women said they had entered their relationship with equal financial assets and 22 per cent said they had more than their spouse or partner, according to the survey.
There is still a gap when it comes to pay, the study found, as only about a third of female respondents said they are their households' primary earners or contribute an equal amount.
The median earnings of women who worked full time was 77 per cent of that for men in 2012, according to the US Census Bureau.
"Women being on equal footing with men in their finances or financial assets is rather rare," said Deborah Soon, senior vice president of strategy and marketing at New York-based Catalyst, which researches women in the workplace.
Women usually get career promotions later than men, which means they lose the value of compounding and fall further behind in building wealth, she said, and they miss opportunities when they leave a job to raise children.
Women make up 4.8 per cent of chief executive officers, 8.1 per cent of top earners and 16.9 per cent of board seats in Fortune 500 companies, according to Catalyst.
The annual nationwide survey of 680 non-US Trust clients was conducted in February and March.
About 43 percent of respondents were women and about a third of all those surveyed had more than US$10 million in investable assets.
Respondents were asked about marital status, holdings and income.
About 547 were married or living with a partner and that group was then asked who brought more assets to the relationship and who was the primary earner.
US Trust didn't have comparative data on these questions from last year, Heilmann said.
Millionaires still have a high allocation to cash, according to the survey, with women more inclined to keep it that way.
Sixty per cent of respondents said they had at least 10 per cent of their money in cash even as yields on savings and money market accounts hover near record lows.
Last year, 56 per cent of those surveyed said they had a large amount in cash.
Investors poured money into assets perceived as safe such as bonds and cash after the 2007-2009 financial crisis, when the Standard & Poor's 500 Index of big US stocks lost more than half its value from peak to trough.
People who fled equities remain risk averse or fear they missed gains that won't continue, Heilmann said.
The S&P 500 surged 47 per cent in the two years ended December 31 as markets rebounded.
US Trust is part of the global wealth and investment management unit of Charlotte, North Carolina-based Bank of America.
It oversees about US$378 billion in client balances and serves those who usually have at least US$3 million in investable assets.