Mr Thanakorn said despite the RT-PCR test showing he had contracted the disease. He had only very mild symptoms, reports Bangkok Post.
The spokesman said he had to work from home and did not specify when the test was carried out.
Prime Minister Prayut Chan-o-cha selected him to replace Anucha Burapachaisri as the government spokesman in August last year. He is known as a staunch defender of the coup leader-turned-premier.
On Mar 26, Thailand registered 25,821 daily Covid-19 cases, 413 fewer than the prior 24 hours, the Public Health Ministry announced.
The daily Covid death toll jumped by 17 to 84 (from the 69 reported on Saturday), all of them Thai nationals between the ages of six and 104.
Of the 25,821 new cases, Bangkok recorded the most - 2,749 - followed by 1,537 in Nakhon Si Thammarat, 1,293 in Chon Buri, 955 in Samut Sakhon and 870 in Samut Prakan.
Just 56 of the new caseload were imported, with the remaining 25,765 transmitted inside the country.