Bipolar disorder is a severe chronic mood disorder that affects up to 4% of the adult population and 1.8% of the pediatric population in the United States. This study will assess how safe and effective Icalcaprant is in treating adult participants with bipolar I or II disorder. Icalcaprant is an investigational drug being developed for the treatment of depressive episodes in adult participants with bipolar I or II disorder. Participants are placed in 1 of 3 groups, called treatment arms. There is a 1 in 3 chance that a participant will be assigned to a placebo. Around 195 adult participants with bipolar I or II disorder will be enrolled in approximately 35 sites across the United States of America. Participants will receive oral capsules of Icalcaprant or matching placebo once daily for 6 weeks, with a 4-week safety follow-up period. There may be a higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.