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.
Born in Zelle am See, Austria, Amar has 31 caps for Bosnia and Herzegovina to his name, three of which were collected at this summer's FIFA World Cup.
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."
