Nisbet, who has 11 caps for his country, was sold to Millwall by Hibernian in summer 2023, signing a three-year contract.
But, despite 30 appearances for the Championship club, 20 of them starts, and scoring five times, he was sent on loan to Aberdeen last season.
“Kevin made a real impact during his time here last season and his arrival provides a long-term solution to our attacking options,” Thelin said.
“He has proven goalscoring ability and we were really starting to see all that Kevin can offer towards the end of last season. As a Scottish international, he helps enhance the identity of our team, which we continue to develop.
“What is most important to us is that Kevin really wanted to be here at Aberdeen. He knows what we are looking to achieve here and he wants to continue to be a part of that.”
Nisbet scored 13 goals in 38 appearances for Thelin’s side, helping them lift the Scottish Cup.
On his return to Millwall, he has made one start and three substitute appearances this season but says “it’s great to be back” at Pittodrie.
“Ever since I left, it was always in the back of my mind that I wanted to come back, but I am here, so I’m delighted,” he said.
“The way last season finished, I felt I was really hitting my best form and I felt at home here.
“The gaffer really helped me improve my game last season. I want to keep improving and this is the best place for it.”
Karlsson, who joined Bologna from AZ Alkmaar, spent last season on loan at Lecce, where he scored one goal in 13 appearances and has been an unused substitute for his parent club this season.
“Jesper is a top professional and to have secured his services is great news,” Thelin said.
“He is an extremely talented footballer who has real quality. “It is important for him to play regularly, and we can provide that opportunity for him to go and perform.”