Rights groups condemned the case, which follows months of protests that have seen unprecedented public criticism of the monarchy and calls for an end to the lese majeste law.
"Today's court verdict is shocking and sends a spine-chilling signal that not only criticisms of the monarchy won't be tolerated, but they will also be severely punished," said Sunai Phasuk of Human Rights Watch.
While the law prohibits insulting or defaming the monarchy, it has regularly been used by authorities and others against opponents.
Actual public criticism of the monarchy had been extremely rare until last year, when months of youth-led protests demanded reforms to the institution, as well as the resignation of Prime Minister General Prayuth Chan-ocha.
At least 40 protesters have been charged under the lese majeste law since November. Up until then, there had been a two-year suspension of the use of the law, with prosecutors instead pursuing protesters and dissidents under other laws, according to the Thai Lawyers for Human Rights.
In June 2020, Prayuth announced that the lese majeste law was no longer being applied because the king had said he didn't want it to be used. At that point, the last prosecution was in March 2018.
Preelert is not the first person to be prosecuted under the law since then. On Tuesday, a freelance writer was sentenced to four years and six months in prison after being convicted under lese majeste and a computer crimes law for insulting the late king – the current king's father – in online articles and poems from 2009 to 2014, according to local media reports.
The case of Preelert, who had been a civil servant for 40 years, dates back to 2014, when Prayuth came to power in a military coup and anti-establishment sentiment was growing.
She uploaded audio recordings by a hardcore critic of the monarchy 26 times on YouTube and three times on Facebook during 2014 and 2015, according to a report in the Bangkok Post.
"I thought it was nothing," Preelert told local media as she arrived at court today. "There were so many people who shared this content and listened to it."