Microsoft was the first U.S. company to exceed a $500 billion market capitalization, reaching the milestone in 1999.
Microsoft’s valuation surged during the late-1990s technology boom, when its Windows operating system and Office productivity software dominated personal computing. The company passed $500 billion in market value in December 1999, near the peak of the dot-com era.
The achievement was based on the price of Microsoft shares multiplied by the company’s outstanding shares. It was a market assessment rather than a balance-sheet total, and the company’s valuation later declined as technology stocks fell and Microsoft faced a major antitrust case.
Microsoft’s 1999 record predates the trillion-dollar milestones of Apple, Alphabet, Amazon, and other modern technology leaders. It is also separate from the question of which company currently has the largest market capitalization.