The 24-year-old full-back joins the Toon in a move worth £29.5 million on a five-year-deal after completing his medical on Tyneside on Monday.
Dedic has reunited with his former manager Matthias Jaissle, after the pair won the Austrian title together at RB Salzburg in 2023.
Speaking to Newcastle United's club website, Dedic 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."
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.
The Bosnian defender has 31 international caps to his name, after featuring three times at this summer's World Cup.
He had also featured against Newcastle United last season in the Champions League for Benfica, and has now made St James' Park his permanent home.
Newcastle will host Liverpool in their Premier League opener this Sunday, and Dedic could be poised to make his debut.