Bipolar disorder (BD) is a major mental disorder that significantly impairs behavior and social functioning. This study assessed the network structure of prodromal symptoms in patients with BD prior to their index mood episode. Semi-structured interviews were conducted with the Bipolar Prodrome Symptom Scale-Retrospective (BPSS-R) to examine patients’ prodromal symptoms. Network analysis was conducted to elucidate inter-relations between prodromal symptoms. A total of 120 eligible patients participated in this study. Network analysis indicated that the observed model was stable. The edge Mania3-Depression9 (‘Racing thoughts’ - ‘Thinking about suicide’, edge weight = 14.919) showed the strongest positive connection in the model, followed by the edge Mania1-depression1 (‘Extremely energetic/active’ - ‘Depressed mood’, edge weight = 14.643). The only negative correlation in the model was for Mania7-depression2 (‘Overly self-confident’ - ‘Tiredness or lack of energy’, edge weight = -1.068). Nodes Mania3 (‘Racing thoughts’), Depression9 (‘Thinking about suicide’), Mania1 (‘Extremely energetic/active’), and Depression1 (‘Depressed mood’) were the most central symptoms. Both depressive and manic or hypomanic symptoms appeared in the prodromal phase. Symptoms reflecting ‘Racing thoughts’, ‘Thinking about suicide’, ‘Extremely energetic/active’, and ‘Depressed mood’ should be thoroughly assessed and targeted as crucial prodromal symptoms in interventions to reduce the risk of BD episodes.