Callamard, who is leading an international inquiry into Khashoggi's death, had a closed-door meeting with Hatice Cengiz at the hotel where she was staying in Istanbul's Şişli district.
Callamard later met with some nongovernmental organization (NGO) representatives.
The U.N. official and the accompanying delegation have reportedly finalized their meetings in Istanbul and are expected to meet with government and intelligence representatives in the capital Ankara.
Callamard was denied entry to the Saudi consulate in Istanbul on Tuesday, forcing her to inspect the grounds while walking around the site.
Despite the unhelpful attitude of the Saudis, Callamard kept her calm, saying, "The request to the Saudi Consulate came quite late, so we need to give them a bit more time to process our request." "We just wanted to [get an] overview of it, and we are respectfully calling on the authorities to give us access," Callamard told reporters.
She met with Istanbul Chief Public Prosecutor Irfan Fidan later Tuesday afternoon.
Callamard arrived in Ankara on Monday, where she met with Foreign Minister Mevlüt Çavuşoğlu and Justice Minister Abdulhamit Gül and will be in the country until Saturday.
The U.N. Human Rights Office of the High Commissioner last week said that the international investigation would be led under Callamard's authorization and it would "review and evaluate, from a human rights perspective, the circumstances surrounding the killing of Khashoggi."
Callamard is being accompanied by Baroness Helena Kennedy, QC, and Professor Duarte Nuno Vieira from the University of Coimbra, Portugal.
Callamard will present her findings and recommendation on the murder investigation as a report in the U.N. Human Rights Council session scheduled for June. "The report will be made public a few weeks before I present it to the Human Rights Council in Geneva, so the end of May possibly," she said.
Khashoggi, a columnist for The Washington Post and a critic of Saudi Crown Prince Mohammed bin Salman, was killed and dismembered by Saudi agents at the kingdom's consulate in Istanbul on Oct. 2, provoking an international outcry.
Saudia Arabia's uncooperative approach has been consistent since Khashoggi was murdered. The kingdom first started with repeated denials, claiming that Khashoggi was not killed at all. After eventually acknowledging the murder, Saudi officials continued their denials this time over the alleged guilt of the royal family and the crown prince, saying that they had no prior knowledge of the murder, although all evidence has pointed to the crown prince as the mastermind. Despite more than 100 days having passed since Khashoggi's assassination, the whereabouts of his body remain unknown.
Ankara has used every means available to bring those responsible to justice while maintaining international pressure. Turkish officials previously said they shared evidence with Saudi Arabia and other nations about Khashoggi's killing and repeatedly called for the suspects to be extradited to Turkey, where the crime was committed. Saudi authorities have denied Turkey's requests and said the suspects will be tried in the kingdom.