主要意见书（英语：Majority opinion），法律术语，源自英美习惯法。它是法律意见书的一种，当法庭中过半数的成员，同意某种法律意见时，所书写的法律文书，法庭将会根据此主要意见书来形成判决；这个主要意见书也会成为未来的判例。但并不是每次的判决都会出现主要意见书，有时，法官把票投给主要判决（例如确认或推翻下级法院的判决），但可能有着截然不同的原因，而不同意同一套主要原因，在这种情况下，可能会出现数个协同意见书，而其中每个都不代表法庭多数成员的意见。因此，参加人数最多的协同意见被称作多数意见书。

In law, a majority opinion is a judicial opinion agreed to by more than half of the members of a court. A majority opinion sets forth the decision of the court and an explanation of the rationale behind the court's decision.

Not all cases have a majority opinion. At times, the justices voting for a majority decision (e.g., to affirm or reverse the lower court's decision) may have drastically different reasons for their votes, and cannot agree on the same set of reasons. In that situation, several concurring opinions may be written, none of which is actually the view of a majority of the members of the court. Therefore, the concurring opinion joined by the greatest number of judges is referred to as the plurality opinion.