Xiaomi launched mi.com in France, Britain and Germany on Tuesday, it said in a statement.
The store went online in the United States on Monday, it added.
Xiaomi, which was only founded in 2010 by entrepreneur Lei Jun, has excelled in China's cut-throat smartphone market by delivering high-performance products at low prices.
In just five years, the Chinese firm has become the world's number three smartphone vendor, behind only Apple and South Korea's Samsung, with rapid growth in emerging countries.
Now the launch of mi.com marks the brand's move into established Western markets.
(Also see: Xiaomi Now Selling Accessories in US and Europe via Mi.com in Beta Test Sale)
A company spokeswoman told AFP on Tuesday that the site is restricted to accessories.
Products on offer include a $14.99 fitness band compared with $99.99 for the mid-range tracker from San Francisco-based Jawbone, $79.99 headphones and battery cells as cheap as $9.99, according to the website.
Shipments are currently limited to the four countries, the company said.
However, Xiaomi has signalled its intention to offer more products to consumers in the US and Europe.
It has enjoyed huge success in its home market of China, and in the first quarter of 2015 held a 13.7 percent market share behind only US rival Apple.
Consumers have snapped up its products which often undercut foreign rivals. Xiaomi's high-end Mi 4 (Review | Pictures) with 64 GB of memory lists for $410, less than half the $998 Chinese retail price of a comparable Apple iPhone 6 (Review | Pictures). Its Redmi 2 (Review | Pictures) budget model is only $115.
It was the top smartphone company in China during 2014, claiming sales of more than 61 million smartphones, up 227 percent from 2013.
Xiaomi has launched products outside mainland China in Hong Kong and Taiwan, as well as southeast Asia, but has yet to expand its footprint into developed countries.
Hugo Barra, vice president of Xiaomi Global, said in the statement he was "truly excited to bring a selection of our best-selling accessories" to consumers in the West.
He added: "We look forward to bringing more Mi products over time to the US and Europe."
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