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Abstract Viscosity is a fundamental physical property that controls lava flow dynamics, runout distance, and velocity, which are critical factors in assessing and mitigating risks associated with effusive eruptions. Natural lava viscosity is driven by a dynamic interplay among melt, crystals, and bubbles in response to the emplacement conditions. These conditions are challenging to replicate in laboratory experiments, yet this remains the most common method for quantifying lava rheology. Few in situ viscosity measurements exist, but none of those constrains the spatial evolution of viscosity along an entire active lava flow field. Here, we present the first real-time, in situ viscosity map of active lava as measured in the field at Litli-Hrútur, Iceland. We precisely measured a lava viscosity increase of over two orders of magnitude, associated with a temperature decrease, crystallinity increase, and vesicularity decrease from near-vent to distal locations, crossing the pāhoehoe–‘a‘ā transition. Our data expand the limited database of three-phase lava viscosity, which is crucial for improvements and validation of the current numerical, experimental, and petrological approaches used to estimate lava viscosity. Further, this study showcases that field viscometry provides a rapid, accurate, and precise assessment of lava viscosity that can be implemented in eruptive response modeling of lava transport.