Stripe Inc. has raised $70 million in a financing round that values the digital-payments startup at $3.5 billion.

The funding is from new and existing investors, said Kelly Sims, a spokeswoman for the San Francisco-based company. The fundraising doubles the value of Stripe since January, when it raised money at a $1.75 billion valuation.

Stripe previously garnered about $140 million from investors including venture capital firms Founders Fund, General Catalyst, Sequoia Capital and Khosla Ventures. Thrive Capital as a new investor in the round, Sims said. The new fundraising was reported yesterday by the Financial Times.

The financing follows a flurry of deals for Stripe as digital payments -- particularly on mobile devices -- take off. In September, Stripe was named as a partner of Apple Inc.’s new mobile-payments service, Apple Pay. Stripe is also powering an e-commerce feature for Twitter Inc. and working with Alibaba Group Holdings Ltd.’s Alipay.

Elite Silicon Valley technology startups are also landing higher valuations as investors clamor for growth. Fourteen U.S. technology companies reached a valuation of at least $1 billion in the first half of the year, more than double the number of startups that did so in all of 2013, according to CB Insights.

Other startups such as Airbnb Inc. and Snapchat Inc. have been valued at $10 billion. Uber Technologies Inc., already the most valuable U.S. technology startup, is in talks to raise at least $1 billion at a valuation of about $35 billion to $40 billion, up from $17 billion in June, people close to the situation said last week.

Stripe was founded in 2009 by Irish brothers Patrick Collison and John Collison. The company offers processing services for online and mobile transactions and provides plenty of code so software programmers can quickly incorporate payment features into their apps.

In an interview in September, Patrick Collison said the company’s focus is on building up its platform.

“We are less than 10 percent of the way there,” he said. “Building a platform that others can build a meaningful business on top of, we are talking about a multidecade horizon.”

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