<p>Cognitive model suggests that social anxiety disorder (SAD) is maintained by persistent biases in attention, interpretation, and memory. Neuroscientific perspectives further implicate that hyperactivation of amygdala and default mode network, alongside prefrontal cortex dysregulation may underlie the neural mechanisms of SAD. This study is the first to investigate the effects of offline high-definition transcranial direct current stimulation (HD-tDCS) on multiple cognitive biases in young adults with social anxiety. Seventy-four university students with elevated social anxiety were randomly assigned to either an active stimulation group (<i>n</i> = 37) or a sham control group (<i>n</i> = 37). Each participant completed ten 20-minute interventions over five consecutive days, with anodal stimulation targeting left dorsolateral prefrontal cortex (DLPFC). Three behavioural tasks were used to assess attention, interpretation, and memory bias pre- and post-intervention. Additionally, participants completed self-report measures of cognitive biases and social anxiety symptoms before and after intervention, as well as at a 1-month follow-up. Mixed-model MANOVA results showed that offline HD-tDCS over the left DLPFC significantly improved attention avoidance, negative interpretation bias, and negative memory bias (<i>F</i> (3,43)&#xa0;= 3.97, η² = 0.22, <i>p</i> = 0.014). However, linear mixed-effects model showed no significant differences between two groups in self-reported symptoms. These findings suggest that HD-tDCS targeting left DLPFC can lead to improvements in behavioural indicators of cognitive biases, with effects persisting at least in the short term after stimulation. However, the benefits may not extend to self-reported conscious experiences. Overall, the results provide promising evidence for the potential of HD-tDCS targeting DLPFC in modulating cognitive biases among young adults with social anxiety symptoms.</p>