<p>This study examined the {332}&lt;113&gt; twinning behaviors in a β-type Ti-15Mo alloy through in-situ tensile tests under a scanning electron microscope equipped with electron backscatter diffraction. Initially, the selection of twinning variants obeyed the Schmid law, with the largest or the second-largest macroscopic Schmid factors (SFs). With increasing strain, certain accommodative twinning variants with negative macroscopic SFs were activated, due to local stress concentrations induced by twin-grain boundary interactions in neighboring grains. Twinning transfer occurred between grains, which was predicted by the geometric compatibility factor (<i>m′</i>) value of twin pairs. In some cases, the <i>m′</i> value of the twin pair was relatively low, but the macroscopic SF was high, indicating the significant role of macroscopic SF in twinning transfer. Twinning transfer sequentially occurred among several neighboring grains, creating a long chain structure of twins. In grains more prone to twinning, twinning-induced strains exceeded the macroscopic tensile strain, whereas grains less susceptible to twinning exhibited minimal strains. The formation of abundant geometrically necessary dislocations along the grain and twin boundaries effectively coordinated the non-uniform deformation both between grains and within their interiors. The occurrence of accommodative twinning variants and twinning transfer was believed to further enhance the deformation ability of the present alloy.</p>