Mykola Melnychenko immediately vowed to see Kuchma and his allies punished for what he says was the ex-president's role in the killing in 2000 of Internet journalist Georgiy Gongadze, Ukraine's most notorious post-Soviet crime.
"I had intended to leave Ukraine for no more than three months," Melnychenko told reporters at Kiev airport.
"But we have now returned. This is our motherland and we will live here. We will do everything in order to ensure Kuchma's gang is held responsible for the dangerous crimes committed against each of you, everything for a court decision."
Melnychenko said he did not want Kuchma's associates, ousted a year ago by the election of liberal Viktor Yushchenko after "Orange Revolution" rallies, to hobble Ukraine's democratic development.
Melnychenko had thrown Ukrainian politics into turmoil by releasing what he said were recordings of conversations between Kuchma and officials made by a device planted under a sofa.
In one such recording, a voice resembling the ex-president's is heard expressing irritation at Gongadze's dispatches and telling his interior minister to "deal with" the reporter.
Melnychenko, wanted on charges of defamation and disclosing states secrets while Kuchma was in power, had lived in the United States since 2001.
After Yushchenko defeated Kuchma's annointed successor in last year's election, he said he was willing to return to testify in murder inquiries, provided he received guarantees of immunity.
No such guarantee has been made publicly, but investigators have said they will listen to Melnychenko's testimony.
Kuchma, whose reputation was tarnished by Gongadze's murder and the mass protests it spawned, has always denied any involvement. The reporter's headless body was found in November 2000, two months after he disappeared.
Police earlier this year announced the arrest of three senior police officers they said had kidnapped and then murdered Gongadze.
But no trial has been ordered and public impatience has grown with the failure to determine who ordered the murder.
Kuchma was summoned to testify before police investigating the murder. His interior minister of the time, Yuri Kravchenko, committed suicide hours before he was due to submit to questioning.