Background and objectives Schema group therapy is a potentially cost-effective treatment for borderline personality disorder (BPD). We piloted the feasibility and effectiveness of a 20-session schema group therapy without individual therapy among psychiatric BPD outpatients in a randomized pilot study registered as a clinical trial (ISRCTN76381242). Methods Altogether 42 psychiatric outpatients diagnosed with BPD were randomized 2:1 to a 20-session weekly schema group therapy plus treatment as usual (TAU) (n = 28) vs. a control group with TAU alone (n = 14). The primary outcome was decline of BPD symptoms in the short Borderline Symptom List (BSL-23) score. Secondary outcomes were decline in symptoms of anxiety, depression, alcohol use, and improvement in functioning and schema modes. Two external experts evaluated validity of the intervention based on videotaped sessions. Results Overall, 23 schema group therapy patients (82%) and 12 controls (86%) completed their treatment. Treatment validity good or very good. However, no significant differences emerged in the primary outcome mean BSL-23 decline (6.95 [SE 5.91] in group schema therapy vs. 12.55 [4.85] in TAU) or in any of the secondary outcome measures. Limitations Despite randomization, the TAU subgroup had non-significantly higher baseline scores in most measures. Small sample size predisposing to type II errors; reliance on self-reported outcomes. Conclusions Schema group therapy was feasible for psychiatric outpatients with BPD. However, in this small pilot study we did not find it more effective than TAU. Effectiveness of this short intervention remains open.