Drywall Ripper is a sliding housing with a cutting mechanism for use with a T-square or the like. The preferred embodiment of the invention utilizes a housing, a slot in the housing with a magnet to hold a blade, clamping components within the housing activated with a clamping button on the housing, a release button on the housing to disengage the clamping components, a viewer opening on the housing, and a groove in the housing for insertion onto a T-square. To use the preferred embodiment of Drywall Ripper, an individual uses the groove to place the housing onto a T-square. The clamping button is then pressed to lock the clamping components of the device at the desired measurement on the T-square. The top of the T-square is then placed on a flat surface parallel to the ground and a piece of drywall is placed perpendicular to the flat surface, supported against its backside. The entire T-square with housing attached is then dragged across the drywall with the top of the T-square still on the flat surface, making a precise cut with the blade at the desired location on the drywall. When the user is finished making the cut, the release button is pressed, allowing the user to move the housing on the T-square to a different measurement if desired.