The Lebanese Yaacoub, 24, said in written testimony read aloud in Greek in a Limassol, Cyprus, courtroom that he had not taken part in plots to target Israeli tourists visiting Cyprus, which prosecutors charge.
"I'm only trained to defend Lebanon," he said.
Yaacoub was arrested in July after writing down license plate numbers of buses transporting Israeli tourists, The New York Times reported Wednesday, two weeks before a bus in Bulgaria was blown up, killing five Israelis and the Bulgarian bus driver.
Bulgarian authorities said the evidence suggested Hezbollah was behind the attack.
Yaacoub admitted in court he had been a Hezbollah member since 2007, but was in Cyprus for purposes of his importing business.
The trial has received little public attention, but a conviction could put pressure on the European Union to declare Hezbollah a terrorist organization, following the lead of the United States, the Times said.