The gap is generational, with 32% of Jewish Millenials identifying as Jewish on the basis of ancestry, ethnicity or culture – compared with 93% of Jews born in 1914-27 who identified on the basis of their faith.
"This shift in Jewish self-identification reflects broader changes in the US public," said Pew's Religion and Public Life Project in a summary of its 210-page report.
"Americans as a whole – not just Jews – increasingly eschew any religious affiliation," with 22% of all Americans identifying with no particular faith, it said.
Nevertheless, 94% of respondents said they were proud to be Jewish, while seven out of 10 felt either very attached or somewhat attached to Israel, a proportion essentially unchanged since the turn of the 21st century, Pew said.
Just 38% felt the Israeli government is making sincere efforts towards peace with the Palestinians, while 44% thought West Bank settlements harmed Israel's security interests.
Pew interviewed 3,475 Jewish Americans by telephone from February 20 through June 13 for its study, giving a statistical margin of error of plus or minus three percentage points.
Release of the findings coincided with a visit to the United States by Prime Minister Benjamin Netanyahu.