It became only the third company to reach the valuation after Apple and Amazon did so last year.
The three tech giants have taken turns in recent months to rank as the world’s most valuable US-listed company.
Microsoft pulled ahead of Apple following third quarter results published after the close of trading on Wednesday evening.
It was up sharply when trading opened on Thursday though some of the gains later faded and it slipped back below the $1tn-mark.
Under chief executive Satya Nadella, the company has spent the past five years shifting from reliance on its once-dominant Windows operating system to selling cloud-based services.
Its flagship cloud product Azure competes with market leader Amazon Web Services to provide a computing platform for businesses. Microsoft also operates the Outlook 365 email service.
The company – founded by Bill Gates in 1975 – today incorporates businesses including the Xbox gaming console, career-focused social networking site LinkedIn, and Surface laptops.
Latest results for the third quarter to the end of March showed revenues up 14% to $30.6bn (£24bn) and profit 19% higher at $8.8bn (£6.8bn).
The figures, boosted by an unexpected uptick for Windows revenue, beat analysts’ expectations.
Investors were also cheered by predictions of further growth for the cloud computing business.
