The boy's efforts to take down the airport's website and infiltrate its computer system on March 22-23 were unsuccessful.
FBI agents questioned a 14-year-old boy from the US city of Pittsburgh, who admitted trying to hack Zaventem airport's website and computer system in March 2016, they said.
"Within the framework of this investigation, the FBI proceeded to a house search in Pittsburgh upon a request for legal assistance from the Belgian Federal Public Prosecutor’s Office, and interrogated a minor of American nationality," the prosecutor's office said in a statement.
"He confessed having committed the acts."
The boy's efforts to take down the airport's website and infiltrate its computer system on March 22-23 were unsuccessful, it added.
"From the investigation and the first analyses of the seized hardware it appeared that there were no terrorist motives for the cyberattacks and that they do not relate at all to the terrorist attacks of 22nd March in Brussels and Zaventem," the statement said.
A source close to the investigation told AFP the minor is a 14 year-old boy.
"He is not radicalised and he told himself it was the right time" to carry out the cyber attack, the source added.
Belgium has been on high alert since three suicide bombers attacked Zaventem Airport and the Brussels metro system in March 2016, killing 32 people.
Investigators said they were carried out by the same Brussels-based cell who staged the Paris attacks that killed 130 people in November 13, 2015.
Both attacks were claimed by the Islamic State group.