Guesmi, with the private Mosaique FM radio station, had appealed a one-year jail term handed down in November before the sentenced was increased to five years under an anti-terrorism law.
Guesmi was found guilty of having taken part in intentionally disclosing "information relating to operations of interception, infiltration, audiovisual surveillance or data collection", said his lawyer Rahal Jallali.
A police officer, found guilty of providing the information to the journalist, was sentenced to 10 years in prison on appeal, up from an original three-year term.
"This is the heaviest sentence pronounced by the Tunisian courts against a journalist," Amira Mohamed, vice-president of Tunisian journalists' union SNJT, told AFP.
"It presents a dangerous authoritarian drift and is a flagrant attack on the freedom of the press."
Guesmi was arrested and held for a week in March 2022, after the Radio Mosaique online service had published a report on the dismantling of a "terrorist cell" and the arrest of its members.
Several local and international rights groups and labor organizations - including the Tunisian League for Human Rights and I Watch - issued a joint statement Tuesday, denouncing the sentence as "a masquerade verdict" and "a major setback for the judicial system".
They warned "against the seriousness of the repressive direction of the current authorities" and called on activists and civil society "to mobilize to defend freedoms and human rights".
These groups have criticized the decline in civic freedoms in Tunisia since President Kais Saied launched a sweeping power grab on July 25, 2021.
In its report published in early May, the journalists union warned of "serious threats" to press freedom in the country.
