All issued U.S. patents are presumed to be valid. However, the USPTO is an administrative agency (i.e., a part of the executive branch of government) and its decisions are therefore reviewable by the courts.
The presumption of validity means that a patentee does not have to prove in court that the patent is valid. The accused infringer bears the burden of proving, by clear and convincing evidence that the PTO erred in awarding the patent.