The 35-year-old Modin had 11 points in 44 games with Columbus and the Los Angeles Kings last season. He signed a one-year contract.
The Thrashers announced the deal with Modin on Monday.
Modin, a native of Sweden, has played 13 seasons, including his first three with Toronto and six years with Tampa Bay. He has 454 career points and has played in 57 playoff games.
He helped Tampa Bay win the 2004 Stanley Cup Championship and won a gold medal with Sweden in the 2006 Olympics.