The Avenue Fonsny (French) or Fonsnylaan (Dutch) is a major street in the Saint-Gilles municipality of Brussels, Belgium.

It connects the Place de la Constitution/Grondwetplein on the south-west of the Small Ring (Brussels' inner ring road) to the municipality of Forest, where it is continued by the Avenue Van Volxem/Van Volxemlaan.

It bears the name of a former mayor of Saint-Gilles, Jean-Toussaint Fonsny.

[1] One entry to Brussels-South railway station is on the Avenue Fonsny.

The street also runs parallel to railway line 96.