Amazon's stock was down 1.14 percent, bringing its market capitalisation to US$690.4 billion, while software maker Microsoft's 1.83 percent fall depressed its market capitalisation to US$690.3 billion.
Bolstered by blockbuster quarterly results, Amazon's stock gained 3.8 percent over the three sessions ended Tuesday, even as Wall Street struggled through a rout that has raised fears that a nine-year bull market may be ending.
Microsoft lost about 3.1 percent over the three sessions ended Tuesday.
Amazon's market cap at points on Wednesday during intraday trade already topped that of Microsoft, but it has never closed at a higher value.
Amazon's explosive growth in retail and cloud computing has sent its shares 77 percent higher over the past year, outpacing Microsoft's 42 percent rise.
(Reporting by Noel Randewich; Editing by Leslie Adler)