In graying Japan, retailers are waking up to a hot new demographic: foreign visitors.

Driven by government tourism promotions and lately a weaker yen, the number of inbound travelers has quietly doubled in the past decade to top 10 million for the first time last year. In 2013, they spent $14 billion on everything from powdered green tea to Prada handbags, to rare, red-coral rings.

With a doubling to 20 million visitors targeted by 2020, the year of the Tokyo Olympics, the growing flow of cash is a welcome turn of events for Prime Minister Shinzo Abe as he seeks to nurse a recovery in consumer spending through a national sales tax hike. It's also sending retailers scrambling to tap into the so-called inbounds windfall through marketing channels from social media to street signs.