At the same time, the number of people who now correctly identify Obama as a Christian has dropped to 34 percent.
Among those who say Obama is a Muslim, 60 percent say they learned about his religion from the media, suggesting that their opinions are fuelled by misinformation, says the poll.
The Survey was conducted before Obama expressed support for the Muslims' right to construct an Islamic mosque near the site of the former WTC.
The belief that Obama is a Muslim has increased most sharply among Republicans, especially conservative Republicans, while the number of independents who say Obama is a Muslim has also increased significantly.
However, there has been little change in the number of Democrats who say Obama is a Muslim, Pew said.
Obama was born to a Kenyan father and an American mother and was brought up by his maternl grandparents in Hawaii because his father, who was also called Barack, left for Africa when Obama was just two. He has said in his autobiography that his grandfather, Hussein Onyango Obama, was a Muslim.