The Sinister Six is a team of super-villains who are members of Spiderman's rogue gallery in the Marvel Universe. The group plays major roles in Disney vs Marvel Villains War and Cartoon Villains War, called The Insidious Six in the latter.
The faction has a minor role in Worst Heroes and Villains War Ever: The Sandbox, being part of the criminal underworld arc, being a somewhat larger group and called the Sinister Six Cartel.