The renewal is being accelerated as Napoli look to shut down interest from the Premier League and Saudi Arabia, having already rejected several approaches this summer.
McTominay has been one of the defining players of Napoli's two-season period of dominance, reaching 80 appearances across all competitions since arriving from Manchester United in 2024.
He scored 13 goals in his first season as part of the Scudetto-winning campaign, and followed that with 14 in the second — numbers that are exceptional for a midfielder and which have made him one of the most coveted players in Europe.
The 29-year-old has spoken consistently about his contentment in Naples.
"It was important to finally play in my natural position as a box-to-box midfielder, where I can both attack and defend," he said.
"I immediately felt comfortable. The manager and coaching staff also helped me enormously to grow and understand the game better. In the end it was the result of daily work."
That desire to remain at the club was underlined when he declined a Saudi proposal worth between £10m and £12m per season, plus bonuses — a package that would have more than doubled his current earnings. The club themselves have rejected at least two Premier League bids.
McTominay's current contract runs to 2028. The proposed extension would tack on two additional years, with an option to extend further to 2031.
Napoli are also to confirm the appointment of Massimiliano Allegri as head coach on a two-year contract following Antonio Conte's departure by mutual consent. McTominay is expected to be a central figure under the former AC Milan and Juventus manager.