Chonthicha, 31, denies the charge of lese majeste and has been granted bail pending an appeal against the decision, lawyer Marisa Pidsaya told Reuters. The court had yet to issue a statement on the sentence.
The law is one of the strictest of its kind in the world and seeks to protect Thailand's powerful monarchy from criticism, with prison terms of up to 15 years for each perceived insult.
Chonthicha is currently a lawmaker for the popular opposition Move Forward Party, which has the most seats in parliament and is facing legal battles of its own over a previous plan to amend the royal insults law, which a court ordered it to drop from its manifesto.
More than 272 people have been charged with insulting the monarchy since 2020, with 17 of those in jail in pre-trial detention, according to legal aid group Thai Lawyers for Human Rights, which compiles data and had defended many of those prosecuted.
Last year, courts sentenced Move Forward lawmaker Rukchanok Srinork to six years in prison over social media posts that criticised the monarchy.
The ruling comes two weeks after activist Netiporn "Bung" Sanesangkhom died in prison, where she was being held in pre-trial detention on charges that include royal insult.
She was on a partial hunger strike, according to Thai Lawyers for Human Rights.
