The funding round raised $18 million, according to the people, who asked not to be named because the deal isn’t public. Menlo Park, California-based Sequoia, which also invested in Stripe’s previous funding, contributed $17 million to the round.

Stripe is vying with EBay Inc.’s PayPal and a variety of startups in the burgeoning online-payment market. Its software lets Web developers take payments via the Internet, without having to set up their own merchant bank account or store consumers’ credit cards. Stripe manages the job of keeping data secure and complying with anti-fraud regulations.

For such an early-stage company, Stripe’s $100 million valuation is “off the charts, statistically,” said Michael J. Patrick, a lawyer at Fenwick & West LLP who works with startups. “There must be something red hot here for Sequoia to invest at that valuation. This isn’t naïve money.”

The average early-stage valuation in the fourth quarter was $7 million, said Patrick, citing data compiled by Dow Jones VentureSource. In 2011, there were 1,414 early-stage deals in the U.S. that raised a total of $8.3 billion, according to the National Venture Capital Association. That’s an average of $5.87 million per deal.

Founded in 2010

John Collison, Stripe’s co-founder, declined to comment. He started the company in mid-2010 with his brother Patrick, who serves as chief executive officer.

The financing adds to the $1.8 million that the brothers raised in seed money shortly after starting the company. That round attracted investments from Sequoia; PayPal co-founders Peter Thiel and Elon Musk; angel investor Ron Conway; and Andreessen Horowitz, a venture firm co-founded by Web browser pioneer Marc Andreessen. Bloomberg LP, the owner of Bloomberg News, is an investor in Andreessen Horowitz.

Max Levchin, another co-founder of PayPal, contributed to the latest funding, one of the people familiar with the matter said. After PayPal, Levchin founded Slide Inc., a social-media company acquired by Google Inc.

PayPal processes around 30 percent of all e-commerce transactions, leaving room for a new player such as Stripe, said Todd Ablowitz, president of Double Diamond Group LLC, a Denver- based payments consulting firm.

“It looks like Stripe makes it very simple to get set up,” Ablowitz said. “If you’re a one or two person company selling coffee cups, God knows you don’t want to be the one storing credit-card information so you can get hacked.”