Top 10 Cities With the Most Millionaires

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If you were looking for millionaires, you'd probably head to London or New York or Monaco. But you might be better off trying Tokyo.

A new study from London-based WealthInsight, the independent research firm, ranked Tokyo the number one city in the world when it comes to millionaire population. The group defines millionaires as those individuals with $1 million or more in assets, not including the value of their primary homes.

The study found that Tokyo has 461,000 millionaires. New York ranked second with 389,000, followed by London (281,000), Paris (219,000) and then Frankfurt (217,000).

Why Tokyo?

The main reason for Tokoyo's ranking is population density, said Andrew Amoils of WealthInsight. While the U.S. has far more millionaires (5 million to Japan's 2 million), America's rich are spread over multiple cities, while Japan's population is more heavily concentrated in its top city.

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More than one fifth of Japan's millionaires are in Tokyo, while New York houses only about 7 percent of America's millionaires.

"It has to do with the concentration," Amoils said.

WealthInsight also looked into the future to see which countries would have the most millionaires. While Japan currently ranks second behind the U.S., it could lose its number two spot to China by 2020.

According to WealthInsight, China could have 3.3 million millionaires by 2020 compared to Japan's relatively flat number of 2.2 million. The U.S. would still tower above both with 7.7 million millionaires.

Maybe Beijing is the next Tokyo.

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—BY CNBC's Robert Frank. Follow him on Twitter: @robtfrank