The 32-year-old, who left the Millers after three seasons for an undisclosed fee in June, is part of Malmo's squad that take on Celtic in the final qualifying round of Europe's elite club competition.
The Swedish side, who are fifth in the Allsvenskan midway through their summer season, travel to Parkhead for the first leg of their tie, meaning a return to Scotland for Arnason following his season-long spell with Aberdeen in 2011.
Victory over the Old Firm giants, the team Millers boss Steve Evans supports, will see Malmo qualify for the group stages of the Champions League.
Arnason has played five times for Age Hareide's side since his Millers exit early this summer, making his debut in the 1-1 draw at Orebro in July.
He scored his first goal for in a 2-1 win over Gothenburg 10 days ago.
And the chance to play at the top level is what attracted Arnason to Sweden.
"I look forward to it and it is obviously an incredibly big tournament," the Iceland international said at his unveiling press conference last month.
"It was a big part of the attraction to come to Malmo, they play at the highest level.
"I am thrilled to get a chance to show what I can do on the big stage again."
Ahead of the game tomorrow, Arnason added in a seperate interview: "I'm glad to get back there again, but now it is more important.
"We do not need to win, just be patient and play the right way. They will play at high speed and try to put pressure on us directly.
"Parkhead is the best arena in the UK, it's like Malmo but with 40,000 more people. They have got great results there, it's helps them tremendously. But it's just about trying to negate that."
Arnason was integral part of the recent success with the Millers, making 130 appearances in a side that went from League Two to Championship survival in the three years he was at the club.
Can Kari cut it at Champions League level? Have your say below.
