Fifty-eight percent replied yes when asked if they thought the use of marijuana should be made legal -- compared to 50 percent two years ago and just 12 percent in 1969, the Gallup polling institute said Tuesday.
Thirty-nine percent said no, and three percent held no opinion.
Support for legalization was predictably stronger among Americans aged 18 through 49, and among Democrats and those who described themselves as independents on the political spectrum, Gallup said.
Its findings mirrored a Pew Research Center poll, released in April, in which 52 percent of Americans said the use of marijuana should be made legal, a rise of 11 percentage points since 2010.
Gallup's findings follow the legalization of marijuana in the western states of Colorado and Washington -- even though possession remains a crime under federal statutes which rank marijuana on a par with heroin.
Other factors behind the trend, Gallup said, include the growing medicinal use of marijuana and a pledge from President Barack Obama's administration not to challenge the sale of marijuana in states that have made it legal.
"Whatever the reasons for Americans' greater acceptance of marijuana, it is likely that this momentum will spur further legalization efforts across the United States," Gallup said.
Supporters of legalization cheered the results, which Gallup said were based on a random sample of 1,028 adults interviewed by telephone on October 3-6, giving a margin of error of plus or minus four percentage points.
"The news of such widespread support for ending marijuana prohibition bodes well for efforts under way to change state laws around the nation," said Rob Kampia of the Marijuana Policy Project, an advocacy group.
"Now, it's time for Congress (in Washington) to act on this clear public mandate by taking action to end the failed experiment of federal marijuana prohibition," added Aaron Smith of the National Cannabis Industry Association.
But Carla Lowe of the California-based Citizens Against Legalizing Marijuana, quoted by NBC News, said she was concerned that poll respondents who favor legalization "are really not aware or knowledgeable about the marijuana that?s out there today."