The report by the Scottish Social Attitudes survey shows 52% of people say they are not religious, compared with 40% in 1999 when the survey began.
Now, the proportion of people who say they belong to the Church of Scotland has fallen from 35% in 1999 to just a fifth (20%).
Other faith groups - including Roman Catholic (15%), other Christian (11%) and non-Christian (2%), have remained steady despite the fall in those who identity as Church of Scotland.
ScotCen Social Research published the findings, it also showed that attendance at religious services is at the lowest level recorded since 1999.
66% of Scots who say they are religious "never or practically never" attend services - that is compared with 49% when the survey began.
Ian Montagu, researcher at ScotCen, said: "Today's findings show that Scottish commitment to religion, both in terms of our willingness to say we belong to a religion and to attend religious services, is in decline.
"However, this change doesn't appear to be affecting all religions equally. Affiliation with the Church of Scotland is in decline while levels of identification with other religions remain relatively unchanged.
"As fewer Scots are acknowledging even a default religious identity, it is affiliation with the national church that is the hardest hit."
The 2015 Scottish Social Attitudes survey interviewed 1,288 people, a representative random probability sample of between July 2015 and January 2016.
