A court in Thailand has jailed a man for two years for selling satirical calendars featuring yellow rubber ducks that prosecutors said defamed the royal family.

The 2021 calendar featured a series of rubber ducks in poses that a Bangkok court ruled resembled Thailand's king, who is protected by some of the world's strictest lese majeste laws, which carry sentences of up to 15 years in prison.

Narathorn Chotmankongsin, 26, was initially given a three-year prison term Tuesday for selling the calendars on the popular pro-democracy Facebook page Ratsadon, according to Thai Lawyers for Human Rights (TLHR).

"But the sentence was commuted to two years without parole after the defendant gave testimony that was beneficial to the consideration," TLHR, a legal group that acts in many lese majeste cases, said in a statement.

The yellow bath toys became an accidental symbol of 2020's pro-democracy protest movement after demonstrators used large inflatable ducks to shield themselves from police tear gas and water cannon.

Rubber-fowl-themed paraphernalia quickly came to dominate street marches, featuring on everything from hats to hair clips.

Thailand's use of lese majeste laws has increased dramatically in recent years, with more than 200 people charged since 2020, according to TLHR.

Human Rights Watch said the court's decision "shows that Thai authorities are now trying to punish any activity they deem to be insulting the monarchy".

"This case sends a message to all Thais, and to the rest of the world, that Thailand is moving further away from -- not closer to -- becoming a rights-respecting democracy," said HRW Asia director Elaine Pearson.