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Abstract Poisson's ratio for earth materials is usually assumed to be positive (V_p/V_s > 1.4). However, this assumption may not be valid in the critical zone because near Earth's surface effective pressures are low (<1 MPa), porosity has a wide range (0%–60%), there are significant texture changes (e.g., unconsolidated vs. fractured media), and saturation ranges from 0% to 100%. We present P‐wave (V_p) and S‐wave (V_s) velocities from seismic refraction profiles collected in weathered crystalline environments in South Carolina and Wyoming. Our data show that ∼20% of the subsurface has negative Poisson's ratios (V_p/V_svalues < 1.4), a conclusion supported by borehole sonic logs. The low V_p/V_svalues are confined to the fractured bedrock and saprolite. Our data support the hypothesis that weathering‐generated microcracks can produce a negative Poisson's ratio and that V_p/V_svalues can thus provide insight into important critical zone weathering processes.