The 26-year-old warhorse had recently agreed terms with the Broncos for a one-year extension until the end of 2026 but a change in family circumstances and a beefed-up offer from the Sea Eagles won his signature.
"Kobe has signed with Manly for four years," Hetherington's manager Chris Haddad told AAP.
"He had previously agreed to terms with the Broncos for next year but his partner has got a very good job in Sydney and Brisbane have given us a release.
"We appreciate the Broncos for understanding things and I thank them," Haddad added.
"Kobe loves the Broncos and he is determined to win a premiership before he goes.
"They brought him there as a kid and now he leaves as an adult."
Haddad said Hetherington had a strong rapport with Manly coach Anthony Seibold who was the head mentor at the Broncos who "turned Kobe from a hooker into a lock".
It was Seibold's successor Kevin Walters who then gave Hetherington his NRL debut in 2021.
Manly had previously offered Hetherington a multi-year deal which he did not accept but they have upped the ante and his family situation changed.
Manly legend Jake Trbojevic has made the No.13 position his own but Hetherington will have a chance to impress and challenge for the spot in future.
The Hetherington camp was not banking on the signing becoming public before the finals series was over.
Haddad said the move would not impact his client's finals campaign with the Broncos.
"Kobe is physically and mentally tough. Opposition clubs say one of the toughest they have ever come across," he said.
Hetherington, who has played 102 games for the Broncos, was given permission by former coach Walters to negotiate with other clubs last season for 2025. Incoming coach Michael Maguire was appointed and made it clear he wanted to keep him.
Hetherington started the season at lock but has been moved to the bench to accommodate the positional switch of Pat Carrigan from prop to No.13.
He has played a key role in inspiring a late season surge into fourth position on the ladder after playing all 24 round games this season.
The son of former Canterbury premiership-winning hooker Jason Hetherington, Kobe is one of captain Adam Reynolds' favourite players.
Reynolds has likened him to a cattle dog because "he has a bit of mongrel and fight in him like a cattle dog and just keeps going".
The genesis of his prowess was in a cattle-yard in rural central Queensland.
"When Kobe and his twin Zac were five years old I would get them to tackle calves," Jason Hetherington told AAP recently.
"I'd be doing all the cattle work and with the calves that I'd brand and let through I'd say, 'go and tackle them boys, wrangle them, jump on them and ride them'. They would get in there and bulldog them and scruff them."
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