Arnason has 13 goals and 28 assists in 60 games this season and should thrive with the talent-rich Senators.
"We were looking for somebody who could play on our second line and we feel he can do that for us," said Senators general manager John Muckler, who finalized the trade just five minutes before the NHL's Thursday deadline.
The 26-year-old Arnason is earning $1.2 million this season and can become a restricted free agent at the end of the season.
He has 57 goals and 82 assists in 245 NHL games -- all with the Blackhawks.
Chicago made three deals before the deadline and swapped veterans for picks in each.