The 24-year-old, who will wear shirt number 37, has penned a five-year deal to become the sixth senior addition of the summer at St. James' Park.
Amar has previous experience working with Magpies head coach Matthias Jaissle, playing under his stewardship in Austria during the early stages of his career with both FC Liefering and Red Bull Salzburg.
Amar said: "It's a special feeling and a really big honour for me to join Newcastle United. I'm really happy to be here and can't wait to start playing for the club and the amazing fans.
"Working with Matthias again was a big factor for me. We know each other well, he knows what I can bring on the pitch and I'm really happy to be playing for him again.
"Playing Premier League football at St. James' Park is going to be really special. Every game feels like a big occasion, with an amazing atmosphere. I'm really excited to get started."
'Defensive and attacking qualities'
Newcastle United head coach, Matthias Jaissle, added: "I'm really happy to add Amar to our squad. He's a player who I know really well, having worked with him from a young age in Austria and followed his career since.
"He will bring both defensive and attacking qualities to our squad, which suits the style of football that we want to play. He's athletic, aggressive and technically very good, but also has a lot of potential to develop further.
"Amar is still young but already has a lot of valuable experience - he's played in the Champions League and the World Cup, as well as representing huge clubs in Austria, France and Portugal. I'm really looking forward to working with him again."
'Perfect fit for our new direction'
Newcastle United sporting director, Ross Wilson, added: "I'm delighted to welcome Amar to Newcastle United. He's a player Matthias knows really well and one we all think very highly of - we see him as a player who strengthens our squad straightaway, capable of performing on the right and on the left.
"He's shown his level at Red Bull Salzburg and Benfica as a modern full-back who has the ability to contribute at both ends of the pitch. He's incredibly hungry to prove himself here at Newcastle United and in the Premier League."
About Amar Dedić
Born in Zelle am See, Austria, Amar's early footballing years were spent with LUV Graz and Sturm Graz. He made the switch to Red Bull Salzburg's youth academy in 2015 and went on to sign professional terms there four years later.
He spent time with their feeder squad Liefering, where he first played under Matthias Jaissle, and also enjoyed a loan stint with Wolfsberger AC, before returning to Salzburg to earn his first-team debut and go on to make 95 appearances in all competitions for the club.
Amar had a loan spell with French giants Marseille during the 2024/25 campaign, making 10 appearances in Ligue 1, before joining Benfica on a permanent basis last summer. He featured for the Portuguese side at St. James' Park in last October's UEFA Champions League clash.
The 24-year-old has 31 Bosnia caps to his name, three of which were collected at this summer's FIFA World Cup.
