State Secrets Protection Act - Declares that in any civil action brought in federal or state court, the government has a privilege to refuse to give information and to prevent any person from giving information only if the government shows that public disclosure of the information that the government seeks to protect would be reasonably likely to cause significant harm to the national defense or the diplomatic relations of the United States.

Requires the court to take steps, which may include in camera and ex parte hearings and other security procedures, to protect sensitive information that comes before it.

Sets forth the court's authority regarding the participation of counsel, appointment of a guardian ad litem to represent an absent litigant's interests, and the disclosure of information when it presents a risk of harm. Provides for court-ordered presentation of adequate or nonprivileged substitutes (redacted copies, summary of information, stipulation of facts) for privileged information.

Allows the government to: (1) assert the privilege in connection with any claim in a civil action to which it is a party, or (2) intervene in a civil action to which it is not a party in order to do so.

Provides that once the government has asserted the privilege, and before the court makes any determinations, the court shall: (1) undertake a preliminary review of the information in question, and (2) provide the government an opportunity to seek protective measures under this Act.

Establishes procedures and a standard for assessing the privilege claim.

Requires the court, if it determines that the privilege is not validly asserted, to issue appropriate orders regarding the disclosure of the information to a nongovernmental party and its admission at trial, with the right to an interlocutory appeal for any such orders. Prohibits such disclosure or admission if the privilege is determined valid.

Grants the courts of appeal jurisdiction of an appeal from a decision or order of a district court determining that the state secrets privilege is not validly asserted, sanctioning a refusal to provide an adequate or nonprivileged substitute, or refusing protective steps sought by the government pending the resolution of the claim of state secrets privilege.