Ad spending on all media outlets fell 5.8% to 34.7 billion baht in the first four months of 2017 as big advertisers monitored their budgets to reduce risk in the second half of the year.
A report by Nielsen Thailand said ad spending on magazines saw the biggest drop, falling 37.6% to 641 million baht in the four-month period, followed by spending on newspapers, down 19.6% at 2.64 billion baht, and radio spending, sliding 18.8% to 1.39 billion baht.
Unexpectedly, internet ad spending fell 11.7% to 484 million baht.
The biggest rise in ad spending was for in-store media, shooting up 52.4% to 320 million baht, followed by cinema, which climbed 27.8% to 2.08 billion baht, and outdoor media, up 22.8% at 1.97 billion baht.
Triluj Navamarat, chairman of the Media Agency Association of Thailand (MAAT), said many advertisers were cautious with their expenditures, even when spending for internet ads, which is the most cost-saving.
"Brands and advertisers are still worried about the economic situation under the military-led government as well as the mourning period before the royal cremation takes place in late October," he said.
Many private companies are carefully investing in all aspects, including advertising, as they feel that the economy is making a slow improvement. Mr Trijul said these companies want to see a clearer pickup in consumer spending and more progress in infrastructure projects.
The unexpected decline of online ads is the lingering result of last year's mourning period as big spenders cut off all media ads during that time, and many of them have not resumed normal ad spending so far.
The print media, including newspapers and magazines, have clearly experienced a sharp drop in ad spending.
Falling ad revenue has hurt many magazines, some of which have shut down or shifted online to ease the financial burden.
According to Nielsen Thailand, big ad spenders in the first four months were Korea King, with 777.46 million baht, up from 557.62 million in the same period last year; Toyota pickups with 314.9 million baht, up from 216.78 million; and Chang with 307.63 million baht, up from 146.95 million.
Last year's total ad spending shrank 11.7% to 108 billion baht because of the sluggish economy, weak consumer spending and the national mourning period.
The MAAT forecasts ad spending on all media this year will grow by 10% to 144 billion baht.