FORTUNE — Founded in 1999, Marcos Galperin’s Argentine startup was the only Latin American dotcom to survive the 2000 crash and go public. “They placed a bet on e-commerce really early on, and it paid off,” says veteran Latin America tech executive Javier Carrique. No kidding. Today MercadoLibre (“free market”) dominates e-commerce in the region, operating sites that match buyers and sellers in 12 countries. MercadoLibre (MELI) has nearly 66 million registered users in Latin America. The company started off as an auction site modeled on eBay (EBAY), which holds an 18% stake. Today 95% of all transactions are fixed price, according to Galperin. MercadoLibre’s marketplace lists an average of more than 10.9 million products for sale daily. The company makes money from marketplace fees, online advertising, and MercadoPago, a secure online payment system inspired by PayPal. Sales hit $299 million last year, up more than 350% since 2007.

Fastest Growing Companies rank: No. 15

HQ: Buenos Aires

Employees: 1,600

The business: E-commerce platform serving 12 Latin American countries

Risk/reward: When Galperin launched the company, only 3% of the regional population had Internet access. Today some 210 million Latin Americans are online, 37% of the population. Another 100 million are expected to plug in over the next five years. Morningstar analyst Dan Su expects MercadoLibre to serve 35% of the Latin American Internet population by 2015, up from 25% in 2010.

But risks abound. MercadoLibre collects revenue in 10 local currencies but reports in U.S. dollars, subjecting it to major currency volatility. Over the years the company has weathered economic and political crises all over Latin America, not to mention the Great Recession. “We all have Ph.D.s in crisis management,” quips Galperin.

This story is from the May 21, 2012 issue of Fortune.