The asymmetric mechanical response and corresponding statistical grain-scale slip/twinning activity for extruded Mg-(0~5 wt%)Y sheets during room-temperature uniaxial tension and compression along the extruded direction were investigated using slip trace analysis and EBSD-based misorientation analysis. The tension-compression asymmetry, in terms of yield strength, ultimate strength and uniform elongation, reduced with the addition of 0.5% Y to Mg. Increasing the Y concentration to 5% resulted in a reversed yield asymmetry (compressive yield strength > tensile yield strength). The non-basal slip activity increased (up to 31%) and the twinning activity decreased with increasing Y content, for both tension and compression. The Y-induced texture change did not significantly affect the pyramidal <c + a> slip, while it promoted basal slip and suppressed prismatic slip. The observed trends of the slip activity suggested that the critical resolved shear stress (CRSS) ratios of CRSSpyramidal <c+a>/CRSSbasal and CRSSpyramidal <c+a>/CRSSprismatic decreased with Y addition. The correlation between macroscopic asymmetric behavior and slip/twinning activity implied that the pronounced pyramidal <c + a> slip, together with the negligible twinning, were responsible for the reversed yield asymmetry. A higher activity of pyramidal <c + a> slip was observed when the slip plane underwent tension compared with that for compression. Display omitted •RT T-C asymmetry of extruded Mg-(0-5)Y sheets was comprehensively and quantitatively investigated.•Statistical grain-scale slip/twinning activity was quantified and correlated to the macroscopic deformation behavior.•Non-basal slip increased, and twinning decreased with increasing Y alloying content.•Enhanced pyramidal slip was observed when the slip plane underwent tension.•The reversed asymmetry was rationalized by the pronounced pyramidal slip and negligible twinning.