Alfredsson signed a one-year contract and he joins numerous Swedish players including Henrik Zetterberg, Niklas Kronwall and Jonathan Ericsson.
Detroit will be in the same division as Ottawa this upcoming season thanks to the NHL's realignment.
It was a surprising move after the Senators said Thursday they were "confident" the 40-year-old Alfredsson would resign.
Alfredsson played 17 seasons with the Senators, including the last 14 as the team's captain. That made him the longest-serving captain in the NHL.
Alfredsson has also been a community leader and he was involved with many charities during his time in Ottawa.
The news was a shock, as you can see in the reaction below.