The Sentinel, released in the United States as The Sentry, is a puzzle video game created by Geoff Crammond, published by Firebird in 1986 for the BBC Micro. It was converted to the Commodore 64 (by Crammond himself), Amstrad CPC (with a cross-compiler written by Crammond), ZX Spectrum (by Mike Follin), Atari ST, Amiga (both by Steve Bak), and IBM PC compatibles (by Mark Roll). The Sentinel was among the first games to use solid-filled 3D graphics on home computers. It won numerous awards upon release and has since appeared on several "best video games of all time" lists.

The IBM PC port supports VGA graphics, with an additional lighting effect: objects and terrain become darker the further away they are from the point of view. The Amiga port has a sampled soundtrack by David Whittaker.

In The Sentinel, the player takes the role of a Synthoid (called just "robot" in the US version), a telepathic robot who has to take control of a number of surreal, checkered landscapes of hills and valleys, by climbing from the lowest spot, where the hunt begins, to the highest platform, over which the Sentinel looms.

The Synthoid itself cannot move across the level; instead it can look around, accumulate energy by absorbing the objects that are scattered across the landscape, create stacks of boulders, generate inert Synthoid shells and transfer its consciousness from one of these clones to another.

List of executable actions:

Controlling Synthoids that are standing at a higher level is fundamental to the game, because only the objects which occupy a visible square can be interacted with (the player may absorb or create objects on a boulder if the sides can be seen). While doing so, the player must watch for the rotation of the Sentinel and be careful not to stand in an area which the Sentinel can see, or else it will start absorbing energy from the Synthoid, and when the energy is gone, the game is over.

Height is gained by placing a boulder on any visible square, and putting a Synthoid on the boulder. The player may then transfer consciousness to the new Synthoid, and absorb the old one. Stacks of boulders of any height may be created, if the player has enough energy. In order to absorb the Sentinel, the player must create a stack of boulders of sufficient height that the Synthoid on top can look down on the Sentinel's platform. When the Sentinel has been absorbed, the player may no longer absorb any energy from the landscape, although objects may be created as normal.

In later levels, the Sentinel is assisted by a number of Sentries. They behave exactly like the Sentinel, but absorbing them is not necessary to complete the level. Unlike the Sentinel, the Sentries do not stand on a platform, but on ordinary squares. If the Sentinel or Sentry cannot see the square the Synthoid is standing on, but its head is visible and there are trees in the vicinity, it may transform one of them into a Meanie, that will force the Synthoid to hyperspace and lose 3 units of energy. If the Meanie cannot see the player's square after a full rotation, it will turn back into a tree and the Sentinel or Sentry will resume rotation.