Those identifying as non-denominational rose from 9 percent in 2000 to 17 percent in 2016.
The number of U.S. Protestants who self-identify with a particular denomination has declined steadily over the past 16 years, according to a Gallup analysis published on July 18.
Fifty percent of respondents identified with a specific denomination in 2000, compared to only 30 percent who did so in 2016.
Those affirming Catholic affiliation moved from 25 percent to 22 percent over the same time period, while those identifying as non-denominational rose from 9 percent to 17 percent.
Baptists have the largest affiliation with 13 percent of all self-identified U.S. Protestants: 10 percent "other Baptists" and 3 percent Southern Baptists.
Four percent of respondents identified as Methodists and an equal number as Lutherans, followed by Presbyterians and Pentecostals (both at 2 percent) and Church of Christ and Episcopal (both at 1 percent).
"Since the percentages of Catholics, Mormons and those who identify with a non-Christian religion have stayed roughly the same over time, this 'rise of the nones' ... has generally been accompanied by an associated decrease in the broad category of Protestants," Gallup explained. "Therefore, there are fewer Protestants of any kind in the American population today, and the pool of those who identify with a specific Protestant denomination is smaller."
The full report is available here.