There is no definitive evidence to suggest that the actor Marlon Brando was diagnosed with bipolar disorder during his lifetime. While Brando was a highly talented and influential actor, and his life was marked by both successes and struggles, there is no public record or credible source indicating that he had a formal diagnosis of bipolar disorder.
Bipolar disorder is a complex and serious mental health condition characterized by periods of depression and mania (or hypomania). If a celebrity's mental health condition is known to the public, it is usually based on either self-disclosure or statements from family members or close associates.
It's essential to avoid speculating about a person's mental health condition without credible information, as doing so can perpetuate stigma and misunderstanding about mental health. As with any historical figure, respecting the individual's privacy and focusing on their contributions and achievements as an actor is more appropriate.