Amazon's competitor to Staples and industrial supply shops now has 1 million customers
Jeff Bezos speaking at the new New York Economic Club luncheon in New York on Oct. 27, 2016.

Amazon isn't just going after retailers and grocery stores. It's capturing a bigger share of the business supplies space as well.

Amazon Business, a business marketplace launched in 2015, now has over one million customers, more than tripling in size from last May, Amazon announced Tuesday. It also said there are more than 85,000 sellers on its platform, up from the 30,000 it had last year.

Amazon Business also offers other services like business analytics and loans to its sellers. The marketplace is only available in the US, UK, and Germany.

The growth in Amazon Business, which sells everything from industrial supplies like screws and pumps to lab and medical equipment, reflects the e-commerce giant's continued expansion. As Amazon looks for the "fourth pillar" of its business — to go alongside the e-commerce marketplace, Amazon Prime and Amazon Web Services — some investors believe Amazon Business could be that next growth driver.

In a report last year, RBC Capital wrote, "The Business Supplies total addressable market is attractive ($5 trillion) and Amazon's offering is unique and leverages Amazon's core competencies. We believe Amazon Business has significant potential to be Amazon's Fourth Pillar," it said.

Besides the numbers of sellers and buyers, Amazon didn't share much data around Amazon Business's growth. Last year, Amazon Business disclosed that it saw over $1 billion in sales in its first year of launch.

Still, investors are keeping a close eye on Amazon's move into the business supplies space. Earlier this month, Susquehanna for Grainger , the industrial parts seller, citing "heightened anxiety about the potential negative impacts of rising online competition from Amazon and other players." Grainger stock is down 24% over the past year.