The ceremony came after a five-hour delay and boisterous protests against ex-President Jimmy Morales.
The 63-year-old Giammattei won the presidency on his fourth attempt in August for Vamos, a party founded in 2017 by politicians, businessmen and military officers on promises of battling poverty and providing better opportunities.
Giammattei was sworn in before several Latin American leaders, among them Juan Orlando Hernández of Honduras, El Salvador President Nayib Bukele, Iván Duque of Colombia and Lenín Moreno of Ecuador. U.S. Homeland Security acting Secretary Chad Wolf and Commerce Secretary Wilbur Ross met privately with Giammattei before the inauguration.
The U.S. Embassy announced via Twitter that the two countries would sign a memorandum of understanding Wednesday on $1 billion in investment in the private sector to stimulate job creation in Guatemala.
One of the early challenges facing Giammattei will be an Asylum Cooperation Agreement signed by his predecessor with the United States government. There was significant opposition to the deal inside Guatemala. The U.S. began sending Honduran and Salvadoran asylum seekers to Guatemala under the agreement in November and recently said it would expand it to Mexicans. A similar deal signed with Honduras could send Guatemalan asylum seekers there.
Giammattei inherits a country in which 59% of Guatemalans live below the poverty line, according to official figures, while nearly 1 million children below age 5 are estimated to live with chronic malnutrition.
He has proposed to build what he calls a "Mayan Train," high-speed rail with a line for cargo and another for passengers. The name mirrors a planned train project for neighboring Mexico that will travel between coastal resorts, cities and Mayan ruins in that country's southeast.
Giammattei will be working without a majority in Congress. His party captured 17 seats.
The surgeon suffers from multiple sclerosis, a disease of the nervous system, and uses crutches to walk. He has also worked as a business consultant in the private sector.
In 2006, Giammattei was the head of the country's prison system when the interior ministry carried out a raid on the Pavon penal farm to regain control from the inmates. Thousands of police, soldiers and armed civilians raided the prison overnight and several inmates died in the operation.
Authorities, bureaucrats and private citizens were arrested, tried and imprisoned for the raid, including Giammattei. After several months in jail awaiting trial he was acquitted and released.
He takes over from Morales who spent much of his four-year term dodging corruption charges.
The former television comedian who campaigned on a promise of "not corrupt, not a thief" will possibly be most remembered for kicking out a U.N. supported anti-corruption mission that was closing in on him and members of his family.
Hours before the inauguration, Morales said he had left the country "stable" and blamed his detractors for attacking him.
Juan Francisco Sandoval, head of the special prosecutor against impunity office, said he hopes the future will be better without Morales. "He was the roadblock for the fight against corruption and impunity," Sandoval said.
In a farewell speech heavy on nationalist and religious themes, Álvaro Arzú, the outgoing president of Congress, said Morales had "defended the sovereignty of Guatemala."