The 29-year-old joined Bulgarian second-tier club Lokomotiv Gorna in February and went on to score twice in 12 appearances, six off the bench.
Moussa had previously been with Tromso, in Norway's top flight, scoring six goals in 23 games.
It is the second signing in 24 hours for manager Neil McCann, who added midfielder Glen Kamara on Wednesday.
In a busy summer transfer window at Dens Park, McCann has also recruited defender Jack Hendry, Dutch winger Randy Wolters, midfielder Roarie Deacon and former Falkirk striker Rory Loy.
Celtic midfielder Scott Allan has also joined on a season-long loan.
Find all the latest football transfers on our dedicated page.
Dundee have signed Tunisian-born striker Sofien Moussa on a two-year contract.