Objective <p>This study aimed to directly compare the effects of synchronous versus sequential tDCS combined with RAGT on lower limb motor function, balance, and walking ability in stroke patients with hemiplegia through a randomized controlled trial.</p> Methods <p>This study was designed as a prospective, single-blind, randomized controlled trial. A total of 60 eligible stroke patients with hemiplegia were recruited and randomly assigned to three groups, with 20 patients in each group. The synchronous group received anodal tDCS stimulation concurrently during lower-limb robotic-assisted gait training (RAGT). The sequential group received tDCS stimulation first, followed immediately by RAGT. The sham stimulation group received sham tDCS stimulation during RAGT. All groups also received conventional rehabilitation training. The intervention was administered five days per week for four weeks. The primary outcome measure was the Fugl-Meyer Assessment of Lower Extremity (FMA-LE) score. Secondary outcome measures included the Berg Balance Scale (BBS) and the Functional Ambulation Category (FAC). All assessments were conducted at baseline (T0), after two weeks of intervention (T1), and after four weeks of intervention (T2) by assessors blinded to group allocation.</p> Results <p>Data analysis of the 54 study completers showed that repeated measures ANOVA indicated significant main effects of time, group, and time × group interactions for both FMA-LE and BBS scores (<i>p</i> &lt; 0.001). Simple effects analysis revealed that at T1, the synchronous group had significantly higher scores than the sequential group (FMA-LE <i>p</i> = 0.002; BBS <i>p</i> = 0.006) and the sham stimulation group (<i>p</i> &lt; 0.001). By T2, the synchronous group’s scores were significantly better than those of the sequential group (<i>p</i> &lt; 0.001), while the sequential group also significantly outperformed the sham stimulation group (<i>p</i> &lt; 0.001). Generalized estimating equations analysis showed that the group, time, and interaction effects for FAC were all significant (<i>p</i> &lt; 0.01), with the synchronous group having the highest cumulative odds ratio (47.893), significantly better than the sequential group (18.379) and the sham stimulation group.</p> Conclusion <p>The combination of tDCS and lower limb robotic-assisted gait training effectively improves lower limb function in stroke patients, and the timing of combination significantly influences the treatment outcome. Specifically, the synchronous combination mode demonstrates faster and greater improvements in lower limb motor function, balance, and walking ability, significantly outperforming the sequential mode, making it a superior “brain-limb collaborative” rehabilitation strategy.</p> <p><i>Trial registration</i> This study was preregistered with the Chinese Clinical Trial Registry (Registration number: ChiCTR2400086567).</p>