SAN FRANCISCO — The world of online marketplaces may already seem crowded with the likes of Amazon.com and the Alibaba Group.

But a new start-up is betting that it can find its own niche — and it has drawn a huge amount of financing to buttress its efforts.

Letgo, which operates a smartphone-focused market for secondhand goods, plans to announce on Thursday that it has raised $100 million in its first major round of venture capital financing

Letgo’s total puts it in the top five biggest such rounds since 2008, according to data from CB Insights. At $100 million, the financing trails only the $225 million raised by AirWatch, a mobile device and app manager; $165 million by Wayfair, another online retailer; and $150 million by OverSee, which runs comparison-shopping sites. Two other companies also raised $100 million. The financing round closed last week.

Backing the company is Naspers, a South African Internet company that has become one of the biggest unsung investors in e-commerce, having backed nascent giants like Tencent and Flipkart.

Now, it is betting on Letgo, which is building what it believes is a next-generation eBay or Craigslist by focusing on the new capabilities of smartphones. The Letgo app, available for iOS and Android devices, prominently features big pictures of the items people are looking to sell.

Unlike predecessors that simply displayed search results in the order they were posted, Letgo draws on users’ locations to order listings. And it features a chat system that makes communication between would-be buyers and sellers much quicker and easier, according to the start-up’s founder, Alec Oxenford.

Together, Mr. Oxenford said, the features should make the market more attractive to so-called millennials whose experience with the Internet revolves around smartphones. Older rivals were built for traditional computers, which younger generations tend to use less, he said.

“It’s the small things combined that make a different user experience that revolutionizes a category,” he said in a telephone interview. Mr. Oxenford declined to disclose other financial information, including the valuation his company fetched in the round.

Classified listings is a business Mr. Oxenford is familiar with: His previous start-up was OLX, a market similar to Craigslist but focused on emerging markets and not on the United States. But he became convinced that smartphones could help transform the business, and he wanted to lead a new and fast-growing company again.

He mentioned his idea to Naspers, which had backed OLX and eventually bought majority control, as part of a plan to solicit money from a range of investors. But Naspers quickly said that it wanted to provide all the capital.

“Alec has distinguished himself as an expert with an extraordinary track record in mobile e-commerce,” Martin Scheepbouwer, the chief executive of Naspers Classifieds, said in a statement. “We have seen firsthand Alec’s ability to execute on his vision as we worked together to build OLX into the global force it is today. In Letgo, Alec has built a platform that is perfectly tailored to the American consumer, and we’re excited to help his team take it to the next level.”

Part of the plan was to move into markets in which Naspers doesn’t have a presence, most notably the United States.

In its brief existence, Letgo has grown quickly, overseeing the sale of more than $25 million in gross merchandise value. The company has garnered over 500,000 listings and claims that its app has been downloaded more than two million times.

The start-up is aiming to grow quickly. Nearly three-quarters of the new round will go toward marketing, according to Mr. Oxenford, with much of the rest going to hire engineers and other new employees.

The enormous size of the round wasn’t dictated by any fears that venture capital financing might dry up soon, given gyrations in the stock market, Mr. Oxenford added. Instead, it was driven solely by the company’s belief that it had a big market opportunity — one that needed an aggressive approach.

“We intend to use the capital, not just to hold it,” he said. “The amount has to do more with the size of the opportunity.”