Fifteen percent of respondents said they had no religion, an increase from 14.2 percent in 2001 and 8.2 percent in 1990, according to the American Religious Identification Survey.
The study found that the numbers of Americans with no religion rose in every state.
"No other religious bloc has kept such a pace in every state," the study's authors said.
In the Northeast, self-identified Catholics made up 36 percent of adults last year, down from 43 percent in 1990. At the same time, however, Catholics grew to about one-third of the adult population in California and Texas, and one-quarter of Floridians, largely due to Latino immigration, according to the research.
Nationally, Catholics remain the largest religious group, with 57 million people saying they belong to the church. The tradition gained 11 million followers since 1990, but its share of the population fell by about a percentage point to 25 percent.
Christians who aren't Catholic also are a declining segment of the country.
In 2008, Christians comprised 76 percent of U.S. adults, compared to about 77 percent in 2001 and about 86 percent in 1990. Researchers said the dwindling ranks of mainline Protestants, including Methodists, Lutherans and Episcopalians, largely explains the shift. Over the last seven years, mainline Protestants dropped from just over 17 percent to 12.9 percent of the population.
The report from The Program on Public Values at Trinity College in Hartford, Connecticut, surveyed 54,461 adults in English or Spanish from February through November of last year. It has a margin of error of plus or minus 0.5 percentage points. The findings are part of a series of studies on American religion by the program that will later look more closely at reasons behind the trends.
The current survey, being released Monday, found traditional organized religion playing less of a role in many lives. Thirty percent of married couples did not have a religious wedding ceremony and 27 percent of respondents said they did not want a religious funeral.
About 12 percent of Americans believe in a higher power but not the personal God at the core of monotheistic faiths. And, since 1990, a slightly greater share of respondents - 1.2 percent - said they were part of new religious movements, including Scientology, Wicca and Santeria.
The study also found signs of a growing influence of churches that either don't belong to a denomination or play down their membership in a religious group.
Respondents who called themselves "non-denominational Christian" grew from 0.1 percent in 1990 to 3.5 percent last year. Congregations that most often use the term are megachurches considered "seeker sensitive." They use rock style music and less structured prayer to attract people who don't usually attend church. Researchers also found a small increase in those who prefer being called evangelical or born-again, rather than claim membership in a denomination.
Evangelical or born-again Americans make up 34 percent of all American adults and 45 percent of all Christians and Catholics, the study found. Researchers found that 18 percent of Catholics consider themselves born-again or evangelical, and nearly 39 percent of mainline Protestants prefer those labels. Many mainline Protestant groups are plagued by conflict over how they should interpret what the Bible says about gay relationships, salvation and other issues.
The percentage of Pentecostals remained mostly steady since 1990 at 3.5 percent, a surprising finding considering the dramatic spread of the tradition worldwide. Pentecostals are known for a spirited form of Christianity that includes speaking in tongues and a belief in modern-day miracles.
Mormon numbers also held steady over the period at 1.4 percent of the population, while the number of Jews who described themselves as religiously observant continued to drop, from 1.8 percent in 1990 to 1.2 percent, or 2.7 million people, last year. Researchers plan a broader survey on people who consider themselves culturally Jewish but aren't religious.
The study found that the percentage of Americans who identified themselves as Muslim grew to 0.6 percent of the population, while growth in Eastern religions such as Buddhism slightly slowed.