A simple, sensitive, and portable viscometer has been designed using an opto-mechanical mouse. The viscosity of a fluid is measured using the infrared light-emitting diodes and the optical diodes of an opto-mechanical mouse. These components are positioned near the top and bottom of a glass tube containing the fluid to be measured. The viscosity of a fluid is based on the time interval between two optical interrupts provided in the two opto-couplers of the mouse during the movement of a ball through the fluid by gravity. This is interpreted in the computer as the change in the "x" and "y" locations of the cursor. In this study, the temperature of the fluid under investigation is controlled using a graphite furnace surrounding the glass tube. In this system, the mouse has the detection systems that determine the sensitivity of the measurement without the need to fabricate any sophisticated electronic and mechanical parts. The reliability of the fabricated viscometer is evaluated by measuring the viscosity of some viscous materials such as ionic liquids. (Contains 2 figures.)