We have built a magnetic needle viscometer capable of measuring the surface shear viscosity of Langmuir monolayers at constant surface pressure or molecular area. A Langmuir trough with dual symmetrical compression/expansion barriers is positioned between two electromagnetic coils that create a homogeneous magnetic field gradient. The gradient drives a Teflon-encapsulated magnetic needle floating on the monolayer along the surface to a terminal velocity. A channel created by two glass plates centers the needle and minimizes drift. The ratio of monolayer surface shear viscosity at a given composition, temperature, surface pressure, and so forth to a reference surface shear viscosity is determined by taking the appropriate ratio of the limiting velocities of the magnetic needle for a given applied force. The monolayer viscometer was calibrated using thin films of silicone oil of known viscosity. We demonstrate the utility of the viscometer by showing that the shear viscosity of saturated dipalmitoylphosphatidylcholine is strongly dependent on temperature and surface pressure and correlates with the liquid expanded to liquid condensed phase transition.