Treating culturally diverse patients (CDPs) presents considerable challenges for psychotherapists, including language barriers, differing beliefs, and insecurities. Improving their transcultural competence requires training, but empirical evidence is lacking. This 6-week randomized controlled trial evaluated the impact of standardized internet-based training on psychotherapists’ transcultural competence (i.e., awareness, engagement, and handling challenges). Demographic data were collected before training. Transcultural competence was measured at pre-training, post-training, and 3-month follow-up. Training satisfaction was assessed at post-training and follow-up visits. In the guided training group (GTG; n = 83), psychotherapists received hands-on training with practical exercises, weekly knowledge assessments, and online feedback. The second condition comprised a non-guided control group (CG; n = 90) that received only text-based training. Primary analyses on both intent-to-treat (n = 173) and completer analyses (n = 95) indicated significant improvements in transcultural awareness and engagement after 6 weeks of training for both groups. Significant within-group improvements were noted, as evidenced by large Cohen’s d effect sizes for both groups. No between-group differences were observed. Qualitative assessments revealed that GTG participants evaluated the training’s concept and content significantly more positively than CG participants and felt significantly less insecure about treating CDPs. Such training could pave the way for the long-term development of innovative, culturally sensitive mental health care services that more effectively meet the needs of CDPs.