7 Video Game Franchises That You Didn’t Know Are Spin-Offs
6. The Portal Series Is A Half-Life Spin-Off
The Portal series is only a Half-Life spin-off due to necessity. The developers that worked on the original Portal were a small team and reusing assets from Half-Life 2 and saying that it takes place in the same universe was just the easiest option.
The games are connected in other ways that solidify Portal as a Half-Life spin-off.
The Half-Life company Black Mesa is established to be a rival to Aperture Science. Black Mesa is mentioned in the song "Still Alive" at the end of the first Portal game. The Borealis which is an Aperture-owned research vessel can be found in Half-Life 2: Episode 2. The dock for the Borealis can be found in Portal 2 which is lacking the ship as that set sailed on over to Half-Life.
So yeah, Portal takes place in the same universe as Half-Life and was released after Half-Life, so by definition, it's a spin-off.
Both franchises are great though and stand perfectly on their own. In the distant future when we get third entries in both franchises, then can we maybe get a proper Portal / Half-Life crossover game?