Bipolar disorder in children, or pediatric bipolar disorder (PBD), is a rare mental disorder similar to bipolar disorder in adults. The diagnosis of bipolar disorder in children has been heavily debated for many reasons, including the potential harmful effects of adult bipolar medication on children. PBD has been proposed as an explanation for periods of extreme shifts in mood, called mood episodes. These shifts alternate between periods of depression or irritability and periods of abnormally elevated moods, called manic or hypomanic episodes. Mixed mood episodes can occur when someone with PBD experiences depressive and manic symptoms simultaneously. PBD mood episodes are different from general shifts in mood experienced by children and adolescents because mood episodes last for long periods of time (i.e. days, weeks, or years) and cause severe disruptions to an individual's life. There are three known forms of PBD: bipolar I, bipolar II, and bipolar disorder not otherwise specified (BD-NOS). The average age of onset of PBD remains unclear, but the reported age of onset ranges from 5 to 19 years old. PBD is typically more severe and has a poorer prognosis than bipolar disorder with onset in late adolescence or adulthood.

Since 1980, the DSM has specified that the criteria for bipolar disorder in adults can also be applied to children with some adjustments based on developmental differences. Genetics and environment are considered risk factors for the development of bipolar disorder, but the exact cause is currently unknown. Diagnosis of bipolar disorder requires evaluation by a professional, and diagnosis of PBD typically requires more in-depth observation due to children's inability to report symptoms properly.

While there is limited understanding of the development of bipolar disorder, research shows that there are many environmental and biological risk factors. Family history is a strong predictor of childhood development of bipolar disorder, with genetics contributing to risk by up to 50%. However, family history does not lead to a certain diagnosis of PBD in a child. Only 6% of children with parents diagnosed with bipolar disorder also have bipolar disorder. Still, children of parents with bipolar disorder should be monitored, especially if they exhibit sleep disturbances and symptoms of anxiety disorders early on. Other factors that can contribute to PBD include substance use disorder and childhood adversity such as abuse or school trauma.

Diagnosis is made based on a clinical interview by a licensed mental health professional. There are no blood tests or imaging to diagnose bipolar disorder. PBD can be difficult to diagnose, especially in children under 11–12 years, as they may be unable to self-assess and communicate symptoms properly. Therefore, it is helpful to obtain information from multiple sources, such as family members and teachers, and use questionnaires and checklists for a more accurate diagnosis. Commonly used assessment tools include the K-SADS (Kiddie Schedule for Affective Disorders and Schizophrenia), the Diagnostic Interview Schedule for Children (DISC), and the Child Mania Rating Scale (CMRS). It is important to assess the child's baseline mood and behavior and determine if symptoms present episodically. Parents are often encouraged to keep mood logs to assist with this. Family history is also important to obtain, as bipolar disorder is heritable. Medication, substance use, and other medical problems should be ruled out to diagnose bipolar disorder accurately.

Early diagnosis and treatment of PBD lead to better outcomes. Anxiety disorders and sleep disturbances often precede mood symptoms. If a child presents with these symptoms alongside major changes in energy and deterioration of function, especially in school, this may warrant evaluation for PBD.

It can be difficult to distinguish PBD due to overlapping symptoms with other conditions such as ADHD, OCD, autism spectrum disorder, depression, anxiety, and conduct disorders. Irritability, distractibility, and poor judgment are symptoms commonly seen in PBD and ADHD. Elated mood and decreased need for sleep can be specifically diagnostic of PBD.

The American Psychiatric Association's DSM-5 and the World Health Organization's ICD-10 use the same criteria to diagnose bipolar disorder in adults and children with some adjustments to account for differences in age and developmental stage, particularly with depressive episodes. For example, the DSM-5 specifies that children may exhibit persistently irritable moods instead of a depressed mood. Additionally, children will more than likely fail to meet their expected body weight instead of presenting with weight loss.

In diagnosing manic episodes, it is important to compare the changes in mood and behavior to the child's baseline mood and behaviors instead of to other children or adults. For example, grandiosity (i.e., unrealistic overestimation of one's intelligence, talent, or abilities) is normal at varying degrees during childhood and adolescence. Therefore, grandiosity is only considered symptomatic of mania in children when the beliefs are held despite being presented with concrete evidence otherwise or when they lead to a child attempting activities that are clearly dangerous, and most importantly, when the beliefs are an obvious change from a child's baseline self-image.