As smartphones finally begin to boom in Japan, so does the cash spent on apps. According to App Annie, the country has overtaken the U.S. as the top grossing country in overall app spending in both the iTunes(s aapl) App Store and Google(s goog) Play this year. Japan’s combined iOS and Android monthly app revenue has more than tripled year-over-year, thanks to a boom in smartphone penetration. It’s also a market dominated by domestic app developers, like GungHo Games and Line. But, most importantly, Japan isn’t finished growing — making it a ripe market for app sales in the years to come.