- Constellation: Canis Major
- Distance from Earth: 8.7 light years
- Star: Sirius (α CMa)
- Moon: Kronor
- Capital: Siriusopolis
- Species: Sirians (extinct)
- Current Leader: Mental (Alias Chaad Sheen ("wizard exterminator" in Sirian), Tah-Um ("The One Infinite" in Eillian))
Sirius has been mentioned in all Serious Sam games. It is the home of many aliens in the series, and the home of Mental, the series' antagonist. It appears to be a very high-tech planet, the most high-tech one. Also, Kronor acts like Earth's moon to Sirius... a fortified moon.
Sirius has a total of 7 levels:
- Siriusopolis Uptown
- Shield Generator
- Be Quick or be Dead
- Welcome to the Jungle
- Jump in the Fire
- Siriusopolis Downtown
- Mental Institution
There is only one boss in Sirius, the Mental Institution. It's only available in the PC version of Serious Sam 2.
- Sirius is another important chapter, like Egypt, since it's Mental's home. Unfortunately (for Serious Sam), Mental is still alive. Kronor is orbiting around it like the Moon is orbiting around Earth. Sirius is a very technological planet and has the highest-tech weaponry and equipment. Though, that's no problem for Serious Sam, because the ones that wield that weaponry and equipment are too....stupid...to use them wisely.