The S&P 500, an index of the 500 largest publicly traded companies on U.S. stock exchanges, surpassed 4,000 points on Thursday for the first time.
It closed at 4,020, up 47 points, or 1.2 percent.
The market milestone is the latest record in a year that has seen unexpected gains despite the global COVID-19 pandemic, which led to some of the worst economic conditions since the Great Depression.
Low interest rates and swift government intervention helped rescue stocks after an initial crash last February and March. Even as the real economy struggled to find its footing amid the ongoing health crisis, securities soared, led by technology companies that were increasingly central to daily life amid lockdowns and health-related restrictions.
Markets, expecting a swift recovery with the advent of vaccines, continued their march ahead.
Warmer weather, an uptick in vaccinations and an additional $2.8 trillion in fiscal support since December accelerated the economic recovery.
But in recent weeks markets have also seen choppy returns, becoming increasingly volatile at the prospect of higher inflation from an overheating economy, which sent bond yields up.
Federal Reserve Chair Jerome Powell has brushed aside concerns over inflation, saying any uptick in prices would likely be muted and temporary.
The Dow Jones Industrial Average, which surpassed 33,000 for the first time in mid-March, closed about 100 points beneath its own recent record at 33,153, a 172 point increase, up 0.5 percent.
Updated at 4:02 p.m.