There were “credible grounds” eight people, including an opposition politician, had forcibly disappeared after criticising the Government, director Hugh Williamson said.
The organisation documented 10 cases since May last year. Another two abductions took place in June. All but one of those men are still missing.
According to Human Rights Watch, four of the missing men all lost their jobs during the state of emergency declared after last year’s failed coup on President Recep Erdogan.
All have alleged links to exiled cleric Fethullah Gulen – who the president accuses of trying to overthrow him.
The cleric’s organisation has been dubbed a terror group and anyone found to have a connection to him may suddenly disappear.
Relatives of those mysteriously disappearing have taken their cases to the European Court of Human Rights, Politico reports.
Those who have disappeared include Onder Asan, a former teacher from Ankara, who later reappeared.
He was dragged out of a taxi by men claiming to be policemen and put into a black Volkswagen van, only, to reemerge six weeks later, alleging that he had had been tortured.
He was known to have taught at a private college with links to Mr Gulen. He was later jailed on terrorism charges.
The other people who were kidnapped were Turgut Capan and Musafa Ozben – who worked at Ankara’s Turgut Ozal University – which was closed down after the coup.
They too were abducted and then forced into a black Volkswagen van.
There are concerns that the disappearances hark back to the 1990s during the state’s brutal war against Kurdish guerrillas, security forces eliminated hundreds of civilians.
Many faced brutal torture, others were found dead.
President Erdogan did not implement these brutal practices, but there are fears that a crackdown is happening again.
Turkey is facing an increasingly tense political climate since the failed coup.
Thirty journalists and newspaper executives are facing life sentences on charges they had links to a failed coup.
German Chancellor Angela Merkel is among the country’s western allies who have criticised Turkey. She called for Ankara’s European Union accession talks to be called off.
Turkey’s Government said it has launched an investigation into the disappearances.
Human Rights Watch is watching closely.
A spokesman said: “I cannot say that there is no effective investigation in this case, but what we know is: There is a deterioration of human rights at a general level and unfortunately there is no effective investigation into torture and mistreatment allegations.”