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Antigorite serpentinite is expected to occur in parts of subduction plate boundaries, and may suppress earthquake slip, but the dominant deformation mechanisms and resultant rheology of antigorite are unclear. An exhumed plate boundary shear zone exposed near Nagasaki, Japan, contains antigorite deformed at 474°C ± 30°C. Observations indicate that a foliation defined by (001) crystal facets developed during plate‐boundary shear. Microstructures indicating grain‐scale dissolution at high‐stress interfaces and precipitation in low‐stress regions suggest that dissolution‐precipitation creep contributed to foliation development. Analysis of crystal orientations indicate a small contribution from dislocation activity. We suggest a frictional‐viscous rheology for antigorite, where dissolution‐precipitation produces a foliation defined by (001) crystal facets and acts to resolve strain incompatibilities, allowing for efficient face‐to‐face sliding between facets. This rheology can not only explain aseismic behavior at ambient plate boundary conditions, but also some of the contrasting behaviors shown by previous field and laboratory studies. Plain Language Summary: The mineral antigorite is important along certain types of tectonic plate boundary, but how antigorite responds to tectonic forces is uncertain. We tried to improve our understanding of this, which might help to better understand how movement occurs between tectonic plates. Rocks that occur along a beach near Nagasaki, Japan, represent an ancient tectonic plate boundary and some of these rocks contain abundant antigorite. We walked along this beach, collecting samples of the antigorite and measuring geological structures. By examining micro‐scale structures within our samples, and comparing with our observations from the beach, we were able to identify the physical mechanisms by which antigorite accommodated movement between the ancient tectonic plates. This mechanism allows movement at a rate that depends on the amount of slip (strain) the rock has accommodated, and the level of driving force, which is related to how fast the slip is. The mechanism we identified provides an explanation for the lack of earthquakes along high‐strain, low driving‐force regions of plate boundaries where antigorite occurs. Key Points: Outcrops of deformed antigorite serpentinite provide a chance to constrain deformation mechanisms at natural shear zone conditionsMicrostructural observations indicate that the dominant deformation mechanisms are dissolution‐precipitation creep and frictional slidingDevelopment of a fine‐grained and foliated fabric with progressive strain leads to reduced effective viscosity [ABSTRACT FROM AUTHOR]