A resistivity-undrained shear strength equation was proposed in this work to investigate the relationship between the undrained shear strength and electrical resistivity of soft Bangkok clay. The field vane shear and screw driving sounding tests were used to evaluate the undrained shear strength of the soft Bangkok clay from 10 field investigations of the canal-side roads. Meanwhile, the electrical resistivity was collected by the low-cost nondestructive resistivity survey method. The relationship between the measured resistivity corresponding to the undrained shear strength was expressed as a linear equation of S u = 7.061ρ with a high statistical correlation (96.20%) between the undrained shear strength and resistivity. The validation between the predicted undrained shear strength from the equation and the measured undrained shear strength from the field sites confirmed the statistically significant relation of data and the reliability of the proposed equation. By using this equation, it successfully predicted the undrained shear strength of the soft Bangkok clay at a depth of 4.00-10.00 m below the ground surface of canal-side roads (zones C, D, and E in the zonation map). The proposed new equation from the resistivity survey in this work, therefore, serves as an alternative tool to fast estimate the shear strength of soft soil in the large area for the preliminary evaluation of road construction.