"The conclusions I would say are fairly grim and reflect what I think is a deep sense of restriction on freedom of opinion and expression throughout the country," David Kaye told reporters on a visit to Ankara.
He said it was clear that Turkey faced threats after the July coup bid - as well as from Islamist and Kurdish militants.
"But this does not mean that the government has, in a sense, a blank cheque to do anything it wants to restrict freedom of expression," he said.
Since a rogue military faction tried to oust President Recep Tayyip Erdogan from power, over 100,000 people within the judiciary, media, military and civil service have been arrested, suspended or sacked.
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Among those arrested include staff from the secular daily Cumhuriyet and the co-leaders of the second biggest opposition Peoples' Democratic Party (HDP).
Kaye is an independent legal expert tasked with reporting back to the UN Human Rights Council. He will present a formal report in spring.
Kaye warned over the long-term impact of the pressure on academics, hundreds of whom have been sacked or dismissed for alleged links to coup plotters or Kurdish rebels.
"The more academics leave, the less you have educators in the country for the next generation."
Kaye met with five of the jailed Cumhuriyet staff, as well as acclaimed translator Necmiye Alpay who is also under arrest.
However he was denied access to internationally-renowned novelist Asli Erdogan as well as influential anti-Erdogan columnist at Cumhuriyet, Kadri Gursel.
The UN said in a statement that Kaye was also unable to meet the arrested International Criminal Court judge Aydin Sefa Akay.
Kaye presented a series of initial recommendations, including a call to release all detained journalists and the repeal of defamation legislation which makes it an offence to insult the president and other public officials.