Pezeshkian won a runoff race against ultraconservative Saeed Jalili on July 5 to replace president Ebrahim Raisi who died in a helicopter crash in May.
Tuesday's ceremony came two days after Iran's supreme leader Ayatollah Ali Khamenei officially endorsed Pezeshkian and gave the 69-year-old heart surgeon presidential powers.
"I as the president, in front of the Holy Koran and the people of Iran, swear to almighty God to be the guardian of the official religion and the Islamic Republic system and the constitution of the country," Pezeshkian said at the ceremony which was broadcast live on state TV.
Pezeshkian, who is expected to unveil his government within two weeks, had secured more than 16 million votes during the runoff, or about 54 percent of the roughly 30 million ballots cast.
Iran's presidential election took place against a backdrop of heightened regional tensions since the Gaza war began in early October, disputes with Western powers over Iran's nuclear programme and domestic discontent over the state of the sanctions-hit economy.
Tuesday's ceremony was attended by senior officials from several countries including Armenia, Tajikistan, Egypt, Sudan, Iraq, Turkey, Saudi Arabia, Azerbaijan, Cuba and Brazil.
European Union envoy Enrique Mora was also present.
Regional Iran-backed allies were also in attendance, namely Hamas leader Ismail Haniyeh and the head of the Islamic Jihad Ziyad al-Nakhalah.
Lebanon's Hezbollah movement was represented by the group's deputy secretary general Naim Qassem while the Yemen's Huthi rebels sent spokesman Mohammed Abdulsalam.
'Death to Israel'
Haniyeh and Nakhalah, whose groups have been fighting Israel in the Gaza Strip since Hamas's October 7 attack on Israel, met with Khamenei and Pezeshkian.
"Supporting the cause of the oppressed Palestinian nation will continue with strength, and no factor can disrupt our will in this direction," said Pezeshkian in a Monday statement.
Tuesday's swearing-in ceremony came amid concerns of war between Israel and Lebanon's Hezbollah following a rocket attack from Lebanon Saturday on the Israeli-annexed Golan Heights.
Israel has accused Hezbollah of responsibility for the attack that killed 12 children but the Iran-backed Lebanese group has denied any involvement.
During Tuesday's ceremony, parliament speaker Mohammad Bagher Ghalibaf gave a speech lambasting what he called Israel's "crimes" in Gaza.
Some Iranians present in parliament chanted: "Death to Israel, Death to America."
Iran has made support for the Palestinian cause a centrepiece of its foreign policy since the 1979 Islamic revolution, and hailed Hamas's October 7 attack on Israel.
Iran's president is not head of state, and the ultimate authority rests with the supreme leader -- a post held by Khamenei for the last 35 years.
On Monday, Pezeshkian warned Israel against attacking Lebanon, saying such an act would have "heavy consequences".
Since his election, Iran's new president has reaffirmed support for the so-called "axis of resistance", Tehran-aligned groups such as Hezbollah and the Huthis, that support Hamas against arch-foe Israel.
Pezeshkian was the only candidate representing Iran's reformist camp allowed to stand in Iran's presidential election, for which all contenders were approved by the conservative-dominated Guardian Council.
During his campaign, the former health minister, had vowed to try and revive a 2015 nuclear deal with world powers which collapsed in 2018 after the United States unilaterally withdrew from agreement.
The deal had given Iran sanctions relief in return for curbs on its nuclear activity.
Pezeshkian has recently called for "constructive relations" with European countries, even though he accused them of reneging on commitments to mitigate the impact of renewed US sanctions.
