The Russian signed a two-year, $10 million US contract through the 2014-15 season.
"Sergei is a high-end offensive defenseman who can quarterback a power play and help generate offense from the blue line," new Stars general manager Jim Nill said in a statement.
Gonchar spent the last three seasons with the Senators. Gonchar had three goals and 24 assists in 45 games the past season, with his assist total fifth best among all NHL blue-liners. He added six assists in Ottawa's 10 playoff games.
The Sens dealt his negotiating rights late last week for what will be a sixth round pick in the upcoming draft.
He's scored 217 goals and 558 assists in 1,177 career regular season games. He leads all active defenceman with 775 points, and is 19th all-time among blue-liners in points.
He won a Stanley Cup in 2009 with Pittsburgh, and prior to that, played for Washington and Boston.