Ministry permanent secretary Areepong Phucha-um said Wednesday that the decline was based on the normal arrival rate of about 85,000 visitors per day, meaning arrivals dropped by 6,000 each day.
Volatility in tourist arrivals might continue in a few weeks, Mr Areepong added.
On Tuesday, the Association of Thai Travel Agents also said 5% of foreign bookings for travel to Thailand had been cancelled since the Aug 17 explosion.
The true facts surrounding impact on tourism from the bomb at the Ratchaprasong intersection that killed 20 people and injured 131 - most of them foreign tourists - contrast sharply with remarks made Tuesday by National Council for Peace and Order spokesman Winthai Suvaree, who tried to play down any suggestion visitors might be put off of Thailand.
"The number of tourists in prominent tourist attractions both in Bangkok and other provinces is still high," Col Winthai said Tuesday in a daily broadcast, without giving any numbers.
"The Ministry of Tourism and Sports has further reported that the statistics of foreign tourists travelling into Thailand is at the normal level."
Data obtained by Reuters from ForwardKeys, which tracks over 14 million travel bookings a day, suggests otherwise.
Its data for the five days after the bombing compared to the same period in 2014 showed bookings to Thailand down 65% and business travel from China tanking 350%.
Skipping Bangkok