The objective of Qix is to fence off, or “claim”, a supermajority of the playfield. At the start of each level, the playing field is a large, empty rectangle, containing the Qix — a sticklike entity that performs graceful but unpredictable motions within the confines of the rectangle.
The player controls a Stix that can move around the edges of the rectangle. Holding down one of the draw buttons allows the marker to move into unclaimed territory in an attempt to create a closed shape. If completed, the captured area (defined as the side opposite of where the Qix is) becomes filled in with a solid color and points are awarded. To complete a level, the player must claim 75% of the playfield (adjustable by the arcade operator to be between 50% and 90%).
The player's marker can move at two different speeds; areas drawn exclusively at the slower speed (orange-red on the screenshot shown) are worth double points. It cannot cross or backtrack along a line in progress.
A life is lost if the Qix touches a line as it is being drawn or if the marker is touched by any of the Sparx – enemies that traverse all playfield edges except uncompleted lines. In addition, if the marker stops while drawing a line, a fuse will appear and burn along the uncompleted line toward the marker; if it reaches the marker, the player loses one life. The fuse disappears once the player moves the marker again. The player has no defenses against the enemies and must out-maneuver them in order to survive.
A meter at the top of the screen counts down to the release of additional Sparx, and the mutation of all Sparx into Super Sparx which can chase the player up an uncompleted line.
After the player completes two levels, the difficulty increases by the inclusion of multiple Qixes, additional Sparx, speed increases, and the eventual appearance of only Super Sparx. In levels with multiple Qixes, the player can also complete the level by splitting the playfield into two regions, each containing at least one Qix.