BANGKOK—A Thai court on Friday has acquitted a prominent singer of the charge of royal defamation, the second time this week a case of such a serious offence has been dropped against an activist.

The Ratchaburi Provincial Court ruled that even though Thanat Thanavacharanon — whose stage name is Tom Dundee — had pleaded guilty, the evidence did not support the charge. He is already serving a prison sentence of 10 years, 10 months after being convicted twice on the same charge. Guilty pleas are often offered in implicit exchange for shorter sentences.

Critics say the law covering royal defamation — also known as lese majeste — is used to punish political dissidents, especially under the military government that took power in 2014. The legal aid group Thai Lawyers for Human Rights said last week at least 162 people have been charged under the lese majeste law since the military seized power.

Lese majeste cases that have gone to trial during the rule of the junta have usually resulted in convictions. However, a human rights lawyer was acquitted of the charge Wednesday, even though he was convicted of sedition. Thanat also was acquitted in March in a third lese majeste case against him.

The authorities, however, have a raft of other laws that can be used against political dissidents, including statutes covering sedition as well as those putting limits on online speech.

Thanat was charged in connection with speeches he made in 2013 at rallies of supporters of former prime minister Thaksin Shinawatra, who was ousted in a 2006 military coup. He was arrested in 2014 on a complaint made by a group opposed to Thaksin.