To correctly analyze debris flow events a rheological approach is necessary, through constitutive modelling the descendent moving mass. Bingham’s model is widely employed in the debris flow literature, its input parameters being plastic viscosity and yield stress. Such rheological approach is important in the evaluation of the propagation of the downhill motion, not only by determining its velocity, but also to aid the delimitation of areas to be affected by such catastrophic type of movement. The present study presents a proposal for the assessment of soil viscosity by laboratory testing on a modified set-up of the well-known slump test. Through the definition of shearing rates in the slump tests, viscosity is determined on a plate–plate rotational rheometer. It was studied one natural soil from Rio de Janeiro, Brazil. Correlations were obtained concerning to soil moisture and shear rates, defined in the slump tests, and also between shear rate and plastic viscosity, the latter defined in the rheometer tests. As a result, good relationships between water content and viscosity were established. Comparing the obtained data with other results available in the literature it is concluded that the developed experimental methodology may provide a simple way to evaluate soils viscosity for debris flow analysis.