Background: A growing body of evidence suggests that pediatric bipolar disorder (PBD) frequently co-occurs with comorbid psychiatric disorders that may impact functioning. Objective: To review existing literature on the prevalence of psychiatric comorbidity and general functioning in patients with a primary diagnosis of PBD. Methods: We performed a systematic literature search on the PubMed, Embase and PsycInfo databases on November 16th, 2022. We included original papers on patients ≤18 years with primary PBD and any comorbid psychiatric disorder, diagnosed according to a validated diagnostic tool. Risk of bias of the individual studies was assessed using the STROBE checklist. We calculated weighted means to assess the comorbidity prevalence. The review complied with PRISMA statement guidelines. Results: Twenty studies with a total study population of 2722 patients with PBD were included (mean age = 12.2 years). We found an overall high prevalence of comorbidity in patients with PBD. The most common comorbidities were attention-deficit-hyperactivity disorder (ADHD) (60%) and oppositional defiant disorder (ODD) (47%). Anxiety disorders, obsessive–compulsive disorder, conduct disorder, tic disorders and substance-related disorders affected between 13.2% and 29% of patients, while one in 10 had comorbid mental retardation or autism spectrum disorder (ASD). The prevalence of comorbid disorders was lower in studies that assessed the current prevalence in patients in full or partial remission. General functioning was overall not specifically decreased in patients with comorbidity. Conclusions: Comorbidity across a broad range of disorders was high in children diagnosed with PBD, especially regarding ADHD, ASD, behavioral and anxiety disorders including OCD. Future original studies should assess current prevalence of comorbidities in patients with PBD who are in remission to obtain more reliable estimates of psychiatric comorbidity in this patient group. The review highlights the clinical and scientific importance of comorbidity in PBD.