Despite volatile financial markets and slow economic growth, the U.S. added 300,000 new millionaires in 2015, bringing the total to a record 10.4 million, according to a new report.

The number of American households with assets of $1 million or more, not including their primary residence, increased 3 percent last year, from 10.1 million, according to Spectrem Group, a market research and consulting firm.

The total marked a new record, but also signaled a slowdown from the 5 percent growth rate in 2014, when climbing stock markets and rising real estate values helped create 500,000 new millionaires.

"Despite ongoing concerns about market volatility and the direction of the U.S. economy, the number of affluent American households expanded in 2015," said George Walper Jr., president of Spectrem Group.

