What is the number of the first test chamber in Portal?

The first test chamber in Portal is Test Chamber 00.

Portal begins with Chell waking inside an Aperture Science relaxation vault before entering Test Chamber 00. This opening chamber introduces the basic structure of the game: GLaDOS gives instructions, portals connect separated spaces, and a Weighted Storage Cube is placed on a large floor button to open the exit.

Test Chamber 00 is deliberately simple because it teaches the player how Aperture’s testing system works before the puzzles become more demanding. After completing it, Chell reaches Test Chamber 01, followed by chambers numbered through 19 in the main campaign. The numbering begins at 00 rather than 01, which is why the answer is easy to misremember.

Another source of confusion is that Test Chamber 00 includes both the relaxation-vault opening and the first small cube puzzle. Some players informally call the relaxation vault a pre-chamber, but the game’s level numbering identifies the opening section as Test Chamber 00. Test Chamber 02 is the first chamber where Chell uses the handheld portal device more actively, not the first chamber overall.