The Leader officially authorized President Hassan Rouhani to take the office for the second time in the ceremony in the capital, Tehran, attended by senior Iranian and foreign officials.
Rohani, a 68-year-old self-proclaimed moderate, won a second term in May.
In a ceremony held on Thursday in Tehran with all top Iranian officials in attendance, Ayatollah Khamenei gave his official approval for the president-elect by giving him a decree to assign him his duties.
After being sworn in on Saturday, Rouhani will have two weeks to present his Cabinet to the Parliament for a vote of confidence.
"Current century is time for moderation; we have no choice but to respect law and moderately move foreward", Rohani said.
Rouhani then hailed nation's turnout in the presidential election calling it "renewing of allegiance with Islamic Revolution".
He said "social justice" and "poverty alleviation" were among the main ideals of the Islamic establishment, adding that the eradication of absolute poverty is one of the objectives of his administration.
But his quest to boost fragile relations with the West into financial infusions to rebuild Iran's oil-based economy has been slowed by investors' fears of pre-existing United States sanctions, and suspicions among powerful followers of Supreme Leader Ayatollah Ali Khamenei of any rise in Western influence. "The government's aim is to improve Iran's image in the world.to safeguard people's rights.to protect the religious democracy and our people's votes", he said. He continued "we are not afraid of enemy's plots and we know how to deal with them; Revolutinary identity anf diligent work would help the country to move ahead despite all countering measures by enemy".