The 28-year-old has impressed manager Craig Brown in training and friendly action this week and is set to sign a contract on Monday.
Arnason, who can play in defence or midfield, had a trial with Hearts earlier this summer after leaving Plymouth.
Brown told RedTV: "I think we have got a good quality international player.
"His preferred choice is midfield but he has played at the back comfortably.
"We have a good array of midfield players and it's just where we need him that we'll play him."
Arnason will be Brown's fifth summer signing and the former Scotland manager feels his side will be harder to beat next term.
"We have stiffened up the team and hopefully it will be a resolute side that plays for Aberdeen this season," he said.
"We found last season the team rolled over a bit too easily.
"We need players with good athleticism and strength of character as well as physical strength.
"We are adding to the strength of the team. Just a little more flair would help now.
"If we can get a striker with flair to add to the good strikers we have, I think we'll have a very competitive squad this season."
Source: Team Talk
Source: Team Talk