The news comes a day before Facebook starts trading as a public company that could be worth more than $100 billion.
Pinterest, an online scrapbook where users can "pin" images and follow others, has grown from less than one million users in May 2011 to about 20 million in April, according to IT research house comScore.
The three-year old company, led by co-founder Ben Silbermann, had been seeking financing for several weeks, but many investors were wary about the billion-dollar valuation, venture capitalists said.
That valuation puts Pinterest beyond the reach of all but a handful of potential acquirers, probably putting it onto a path for an eventual IPO rather than an acquisition.
Still, the company attracted many interested parties.
"It was one of the most hotly contested and sought-after financing events in Silicon Valley in quite some time," Mike Jaconi, a Rakuten executive who was involved in the negotiations, told Reuters.
Jaconi said Rakuten's presence in 18 countries helped it get a piece of Pinterest as the U.S. company was looking to expand internationally.
Rakuten's CEO Hiroshi Mikitani will take on an advisory role at Pinterest, Jaconi said.
A Pinterest spokeswoman said the Japanese company would integrate the "pin it" button on its websites.
Rakuten, which offers services ranging from online golf reservations to internet banking, was joined in the investment by existing Pinterest shareholders Andreessen Horowitz, Bessemer Venture Partners and FirstMark Capital.
Rakuten is aggressively expanding into global markets. Last year, it bought Canadian ebook company Kobo for $315 million and UK online shopping site Play.com for about 25 million pounds.
High valuations
Valuations for consumer-oriented tech startups have been rising dramatically, as illustrated by Facebook's $1 billion purchase of photo-sharing application Instagram last month.
Pinterest's $1.5 billion valuation is up sharply from $200 million late last year.
The company, like Instagram, has no significant revenue.
A Pinterest spokeswoman declined to disclose the company's revenue, saying it is not focused on monetization for now.
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