He returned to Beirut on Tuesday and met Lebanese President Michel Aoun at Baabda Palace alongside security chief Abbas Ibrahim.
An information technology expert, Zakka disappeared in Tehran while attending a state-sponsored conference in September 2015.
Speaking to reporters on Tuesday, Zakka praised Lebanese government officials and his family for securing his freedom.
“The campaign for my release started in Lebanon, developed in Lebanon, and ended in Lebanon,” he said, adding that he would not discuss the details of his “kidnapping, detention, and false accusations”.
He condemned “speculation” related to the negotiations of his release.
Some Iranian media outlets have described him as a US spy, while other media have speculated that his release may be part of a wider deal between Washington and Tehran.
Security chief Ibrahim also addressed reporters on Tuesday and thanked the political leaders in Beirut and Tehran for reaching an agreement.
“I’d like to thank His Excellency President Rouhani and the Iranian authorities for responding [to our calls].”
The US State Department, which had said he was being unfairly held and called for Zakka to be freed, applauded the release on Tuesday.
“It is without a doubt a great day for Mr Zakka, his family, and all those who have supported him during his unlawful imprisonment,” a State Department spokeswoman said.
“We hope that Mr Zakka’s release is a positive sign for American detainees in Iran,” she said.
Zakka vanished in Iran after attending a conference there in September 2015. According to a statement by his lawyer, Zakka was last seen leaving his hotel in a taxi to the airport to return to Beirut. But he never boarded his flight. In November 2015, Iranian state television announced Zakka was in Iranian custody and accused of espionage.
The statement claimed that Zakka, who graduated from the Riverside Military Academy of Gainesville in Georgia, had “deep links” with US military and intelligence agencies.
In 2016, he was sentenced to 10 years in prison and ordered to pay a fine of $4.2m for espionage.
In the statement published on Iran’s Mizan News Agency on Tuesday, Iran judiciary spokesperson Gholam Hussein Esmaeili said Zakka’s release was in line with Iran’s Constitution, which allows for the conditional release of prisoners sentenced up to 10 years, if they had served at least a third of the sentence and shown good behaviour.
The spokesperson also mentioned key ally Hezbollah’s request to expedite his release. The conditions of Zakka’s release were not specified.
Paula Yacoubian, an independent MP who sits on the Lebanese parliament’s foreign affairs committee, described Zakka’s case as “mysterious”.
“I hope that things will be clearer later on, and we can really understand what happened with him and whether he’s innocent or not,” she told Al Jazeera.
Tara Sepehri Far, Iran Researcher at Human Rights Watch (HRW), welcomed his release and said his arrest was part of a pattern of detentions of foreign nationals and dual citizens that were “politically motivated.”
“The court sessions behind closed doors, [and] there’s very little transparency,” Sepehri Far told Al Jazeera. “The branch of the court … is known for its lack of fair trial standards and blatantly ignoring due process standards.”
She added that they have documented several cases of detainees who brought accusations of torture that were “blatantly ignored”.
Zakka was held at Tehran’s Evin prison, a facility established in 1972. HRW and other rights groups have reported allegations of torture and ill-treatment there, including solitary confinement and denial of access to medical care.
Zakka’s family and friends, who have been campaigning for his release, have claimed he went on hunger strike numerous times and was tortured.
Sarah Fallah, a Lebanese lawyer who represented Zakka, told HRW in March 2016 that Iranian authorities refused to let her visit her client.
Over the past four years, Zakka’s family members have repeatedly called on the Lebanese government to negotiate for his release.
The US government was also vocal in calling for Zakka’s release. Both the Congress and the Senate passed resolutions in 2017 calling for the unconditional release of US citizens and residents held in Iran, including the Lebanese national.
More recently, in December 2018, then-US Ambassador to the United Nations Nikki Haley endorsed and republished a letter by several families of held US nationals and residents in Iran, including Zakka’s relatives.