It is a huge show of faith by the 27-year-old, with the new contract to cover NRL seasons 2018-2021.
McCullough is already in the club leadership group, and is considered future captaincy material.
As he signed the deal today he said he was pleased to lock in his long-term future with the Broncos.
“It is exciting,” said McCullough after he inked the contract.
“To be here another four years and to have that security for myself and my fiancée is really good, and something that doesn’t happen often in rugby league.
“But I am very grateful for it.”
Broncos’ chief executive Paul White said securing McCullough was further good news for the Brisbane club.
“Andrew has been with us since his early teens so to have him re-sign for another four years is wonderful news for the club,” said White.
“We see these guys as the next crop of leaders of the Broncos, so to have those players committing to new deals underlines the great stability of the club as a whole.”