Viscosity is an important property in the crystallisation process of lactose from supersaturated solutions during lactose production. Viscosity, however, is difficult to measure for supersaturated solutions by conventional, invasive, rheological techniques. To overcome this issue, dynamic light scattering (DLS) was used, whereby latex particles were added to the sample as a tracer. From the known size and measured diffusion coefficient of the latex particles, viscosity of lactose solutions could be determined as a function of temperature (20–80 °C), lactose concentration (10–50 g α-lactose monohydrate 100 g−1 solution) and degree of supersaturation (−40 to +25 g α-lactose 100 g−1). When viscosity was expressed as a function of degree of supersaturation of α-lactose, curves at different temperatures collapsed onto a single master-curve. The results highlight the potential of DLS with latex tracer particles as a convenient and reliable tool for measuring viscosity of even strongly supersaturated solutions of lactose.