Background: A subgroup of those with bipolar spectrum disorders experience ongoing mood fluctuations outside of full episodes. We conducted a randomised, controlled feasibility study of a Dialectical Behavioural Therapy-informed approach for bipolar mood fluctuations (Therapy for Inter-episode mood Variability in Bipolar [ThrIVe-B]). Our study aimed to examine the feasibility and acceptability of a future definitive trial evaluating the clinical and cost effectiveness of the ThrIVe-B programme. Participants were required to meet diagnostic criteria for a bipolar spectrum disorder and report frequent mood swings outside of acute episodes. They were randomised to treatment as usual (control arm) or the ThrIVe-B intervention plus treatment as usual (intervention arm). Follow-up points were at 3, 6, 9 and 15 months after baseline, with 9 months as the primary end point. To evaluate feasibility and acceptability we examined recruitment and retention rates, completion rates for study measures, adverse events and feedback from participants on their experience of study participation and therapy. Results: Of the target 48 participants, 43 were recruited (22 in the intervention arm; 21 in the control arm), with a recruitment rate of 3.9 participants per month. At 9 months 74% of participants engaged in research follow-up assessment, exceeding the pre-specified criterion of 60%. There were no serious concerns about the safety of the research procedures or the intervention. On one of the four candidate primary outcome measures, the 95% CI for the between-group mean difference score excluded the null effect and included the minimal clinically important difference, favouring the intervention arm, whilst on no measure was there evidence of deterioration in the intervention arm relative to the control arm. Attendance of the intervention (50% attending at least half of the mandatory sessions) was below the pre-specified continuation criterion of 60%, and qualitative feedback from participants indicated areas that may have hampered or facilitated engagement. Conclusions: It is broadly feasible to conduct a trial of this design within the population of people with frequent bipolar mood swings. Changes should be made to the therapy to increase uptake, such as simplifying content and considering individual rather than group delivery. Trial registration ISRCTN: ISRCTN54234300. Registered 14th July 2017, http://www.isrctn.com/ISRCTN54234300.