Somsak Jeamteerasakul, a Bangkok university lecturer who has called for reform of the monarchy, turned himself in to hear a lese majeste complaint against him filed by a Thai army officer.
"He came to hear the charges and the investigator released him without bail," Police Colonel Mana Phaochauy said.
He declined to discuss details of the complaint against Somsak, a history lecturer at Bangkok's prestigious Thammasat University.
But Somsak told AFP that the basis of the charges was a two-part article he wrote about the king's youngest daughter, Thai Princess Chulabhorn, who he believes is not protected by the lese majeste rules.
"I deny all the charges and will submit my written defence in two to three weeks," he said.
The academic has tested a long-standing taboo with his call for reform of the royal family, although he has said his proposals do not envisage the abolition of the monarchy.
Public prosecutors will decide whether the case goes to court based on evidence collected by the police.
Lese majeste is a serious offence in Thailand, punishable by up to 15 years in jail.
The royal family is an extremely sensitive subject in the politically divided nation, which is preparing for a tough election fight on July 3.
More than 50 Thai and foreign academics living outside the kingdom signed an open letter released Tuesday, protesting against what they called "the latest signal of the worsening atmosphere of freedom of expression in Thailand".
Up to about 50 members of the opposition "Red Shirt" protest movement also gathered at the police station in support of the lecturer.
Thailand has drawn flak from rights groups for suppressing freedom of speech using lese majeste legislation, which bans criticism of the royal family and the institution.
King Bhumibol Adulyadej, 83, the world's longest-reigning monarch and revered as a demi-god by many Thais, has been in hospital since September 2009.
Thailand's army chief has warned political figures to keep the monarchy out of debate ahead of the general election, the first since deadly political violence rocked Bangkok last year.