More than 68 million visits were made between January to November last year.
But the figures from the Great Britain Day Visits Survey (GBDS) showed this was down 19% on those months in 2014.
However, the Welsh government said domestic overnight trips to Wales for the first nine months of 2015 increased by 1.8% on the previous year.
For day trips, the survey also showed visits generated expenditure of Â£2.4bn in 2015, down 1% on the previous year.
In contrast, spending on overnight visits to Wales between January to September last year rose by 12% compared to the same period in 2014.
A Welsh government spokeswoman said the fall in day visits was "not unique to Wales".
"Although the number of trips is down 19%,  the spend is only down 1%, compared to a record breaking year in 2014," she said.
"Meanwhile, domestic overnight figures for the first nine months of 2015 show an increase of 2% in the number of trips and 12% in expenditure, which means that more people are staying in Wales longer and spending more, which is excellent news for Wales' economy.
"This also suggests the decrease in volume of day visitors, as reported in the Great Britain Day Visits Survey may in part be due to visitors taking longer trips instead of day visits."
The number of tourist day visits to Wales from the rest of the UK dipped nearly 20% in 2015 compared with the previous year, new figures have shown.