Barnes, who still had two years remaining on his existing deal, put pen to paper on fresh terms after making 57 appearances across all competitions last season and finishing with a career-high 16 goals.
Since joining from Leicester City in the summer of 2023, the winger has racked up 30 goals in 120 appearances for his adopted club, a return that few wide players in the Premier League can match, and was rewarded with a first England call-up in nearly six years when he featured in March's friendly against Uruguay at Wembley.
His extension arrives amid a summer that has seen Newcastle lose Anthony Gordon to Barcelona and Sandro Tonali linked with a move away, making the 28-year-old's commitment a timely boost for Eddie Howe's squad.
"It feels great to sign this new contract so early before the new season starts. It's been an amazing journey so far and I'm delighted to continue that," Barnes said.
"When I first joined the club I quickly learned what a special place it is, but after three years here I know that even more.
"I love everything about it – the city, the stadium, the people – and I'm really happy extending my time here."
Barnes is best remembered by supporters for the latest winning goal in Premier League history, a 102nd-minute strike that beat Leeds United at St James' Park earlier this year.
Howe described the new deal as thoroughly deserved: "Harvey's new contract is very well deserved and a reflection of the work he's put in and the quality he's shown throughout his three years at Newcastle United. He's calm in front of goal, whether that's one-on-one or from distance, and is always a threat from wide areas."
Sporting Director Ross Wilson, for whom the deal is his third major contract renewal after Fabian Schär and Sven Botman, added: "Harvey's record for the club speaks for itself. We see him as a big part of our plans going forward, and it made total sense to secure his future."
At the other end of the club's business, defender Ciaran Thompson has completed a move to National League North side Darlington after his release from St James' Park this summer, joining a club managed by former Magpies defender Steve Watson following a successful trial.
The 21-year-old, who is Blyth-born, captained Newcastle's under-21s and featured in the National League Cup, EFL Trophy and UEFA Youth League during a near-decade in the club's academy system, having arrived as a 13-year-old and eventually put pen to paper on his first professional deal in July 2023. He is one of seven academy players to leave Newcastle this summer as their contracts expired.
Darlington assistant manager Terry Mitchell said: "We're really delighted to get Ciaran on board. He'll really add to our quality and has great versatility. He can play anywhere across the back four, at wing back, and can also drop in to a deep midfield role if needed."
Thompson could make his competitive debut for the Quakers when they host Worksop Town on Saturday 8 August.