Morsi, a leading member of the Muslim Brotherhood group, which is now banned in Egypt, suffered a fatal heart attack in a Cairo court on Monday while on trial on espionage charges.
"I believe the United Nations will put Morsi's suspicious death on its agenda and hold those responsible accountable," Erdogan said in a speech at a rare news conference with foreign journalists in Istanbul.
Erdogan has vowed to seek the Egyptian government's "trial" in international courts over Morsi's death, calling on the Organization of Islamic Cooperation to act. He also called Morsi a "martyr" and said he did not believe he died due to natural causes.
Egypt's foreign ministry denounced Erdogan's comments on Thursday, calling them "crude violations" against Egypt.
The 67-year-old Morsi, the first democratically elected head of state in Egypt's modern history, had been in jail since the army commanded by Egypt's current president Abdel Fattah al-Sisi toppled him in 2013 after barely a year in power following mass protests against his rule.
Erdogan's Islamist-rooted AK Party supported Morsi’s short-lived Egyptian government, and many Brotherhood members and supporters have fled to Turkey since its activities were banned in Egypt.