(Reuters) - Social networking site Pinterest has raised $100 million from a group of investors led by Japanese online retailing giant Rakuten Inc in a deal that reportedly values the three-year-old U.S. company at $1.5 billion.

Rakuten said it was joined in the capital-raising by existing shareholders Andreessen Horowitz, Bessemer Venture Partners, FirstMark Capital and a number of angel investors.

The new round of financing values Pinterest - one of Silicon Valley's fastest growing startups - at $1.5 billion, up sharply from $200 million late last year, Wall Street Journal said.

Valuations for tech startups have been rising dramatically, as illustrated by Facebook's $1 billion purchase of photo-sharing application Instagram last month.

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 comScore. It is the 16th most-visited site in the United States, according to Web information company Alexa.

The Palo Alto, California-based company, which was launched in the fall of 2009, is led by co-founder Ben Silberman.

The funding will help Pinterest expand into Japan and Rakuten's 17 other markets, Rakuten said.

"We see tremendous synergies between Pinterest's vision and Rakuten's model for e-commerce," Rakuten Chief Executive Hiroshi Mikitani said in a statement.

--