Tantawi, who had hoped to run against President Abdel Fattah al-Sisi in elections last year, was found guilty of election campaign irregularities in February.
"The sentence at the time was suspended on bail until the appeal today," Elganadi said, adding that Tantawi was arrested as soon as the decision was declared.
The former lawmaker was also "barred from running in parliamentary elections for five years," according to human rights group the Egyptian Commission for Rights and Freedoms.
The Matareya Misdemeanour Court also upheld sentences against 22 members of Tantawi's campaign team, including its director Mohamed Aboul Deyar, of "one year in prison with hard labor," Elganadi said.
They had been convicted of "circulating election-related papers without official authorization" in the lead-up to the election, which Sisi won in his third landslide victory.
Tantawi had accused authorities of hampering his effort to collect the endorsements required to run in the presidential election, under various pretexts including computer malfunctions.
Tantawi instead asked his supporters to fill out unofficial "popular endorsement" forms — a tactic the authorities labeled as tantamount to election fraud.
He ultimately collected only 14,000 endorsements — well short of the 25,000 needed from at least 15 of Egypt's 27 governorates to enable him to run.
Alternatively, he would have had to garner nominations from at least 20 parliamentary deputies.
The former member of parliament withdrew his candidacy before the December vote, citing harassment and obstruction.
The National Election Authority announced Sisi's victory on December 18 with 89.6 percent of the vote.
He had run against three relatively unknowns: Hazem Omar of the Republican People's Party, Farid Zahran (Egyptian Social Democratic Party) and Abdel-Sanad Yamama (Wafd Party).
According to Human Rights Watch, the authorities deployed "an array of repressive tools to eliminate potential challengers," including jailing another prospective candidate, Hisham Kassem.
Cairo has long been criticised for its human rights record, with rights groups estimating that tens of thousands of political prisoners remain behind bars, many of them in brutal conditions.
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