<p>This study proposes a novel partially nondestructive Shear Test Method (STM) for estimating the compressive strength of in-situ masonry mortar. Unlike conventional nondestructive (rebound, penetration) and partially destructive (point load, local compression) methods, STM applies controlled near-pure shear stress to thin mortar sheets extracted from masonry joints, thereby enabling a more direct mechanical correlation with compressive strength. A portable test apparatus was developed, and three experimental series were conducted to evaluate repeatability, establish empirical correlations with cube strength, and compare STM with other PDT/NDT techniques. The results show a strong correlation between STM shear strength and cube compressive strength (R² = 0.96, confidence interval within ± 10%), with coefficients of variation typically below 10%. In-situ verification further demonstrated that STM can detect spatial variability in existing buildings, although surface deterioration and moisture heterogeneity may influence accuracy. Finite element analysis confirmed the stress distribution and failure process, supporting the experimental findings. STM thus provides a rapid, accurate, and minimally invasive alternative for assessing mortar strength in practice, though further validation across mortar types and environmental conditions is required.</p>