The NFL Network said that Garoppolo and the 49ers have agreed a deal with a value of $137.5 million through to 2023.
But the talented 26-year-old was sensationally traded to San Francisco last October.
Garoppolo has only started seven NFL games in his career -- two with the Patriots and five with the 49ers -- but has won all of them.
He sparked a late revival after joining the 49ers, guiding them to five consecutive victories to finish the season at 6-10 after being 1-10 when he arrived.
The 49ers are expected to announce the contract at a press conference on Friday.