BACKGROUND: Options for 'treatment-resistant bipolar depression' (TRBD) are limited. Two small, short-term, trials of pramipexole suggest it might be an option. AIMS: To evaluate the clinical effectiveness and safety of pramipexole in the management of TRBD. METHODS: A multi-centre randomised, double-blind controlled trial including participants ⩾18 years old with TRBD (failure to respond, tolerate or clinical contraindication/patient refusal of ⩾2 of quetiapine, olanzapine, lamotrigine or lurasidone) randomised 1:1 to pramipexole (max 2.5 mg/day salt weight) or placebo added to ongoing mood stabiliser ( RESULTS: Pramipexole ( CONCLUSIONS: Clinically large, but statistically non-significant, effects of pramipexole on depression at 12 weeks, with significant longer-term benefits on mood and function were observed. Pramipexole use was complicated by dose titration and increased hypomanic symptoms. The small sample size limits interpretation. Furthermore, larger randomised placebo-controlled trials are warranted.