The nationwide poll from the Pew Research Center shows 52 percent support making pot legal while 45 percent are against it.
Support has been on the rise, but spiked 10 percentage points since 2010.
The new finding marks the first time in 40 years of polling that a majority of Americans have supported legalization.
Just one decade ago, only a third of American adults backed making marijuana legal.
In 1969, just 12 percent favored legalizing marijuana use, according to the poll.
Support is highest among millenials, those born after 1980, but is also on the rise among baby boomers.
Half of baby boomers surveyed now favor legalization. Support is also up among Hispanics and moderate Republicans, the poll showed.
Just one-third of Americans now believe smoking marijuana is "morally wrong," according to the poll, while 50 percent say it's not a moral issue.