Sunshine for Regulatory Decrees and Settlements Act of 2021 This bill establishes transparency and public accountability standards for federal agencies with respect to (1) certain civil actions seeking to compel agency action, and (2) related consent decrees and settlement agreements. For example, an agency must publish a complaint filed against it within 15 days. Additionally, settlement proceedings must be conducted through mediation or an alternative dispute resolution program of the court, and those proceedings must include intervening parties. The bill also creates a presumption in favor of a motion to intervene in settlement proceedings. Further, at least 60 days prior to entering a consent decree or settlement agreement, an agency must publish, and accept and respond to public comment on, the proposed agreement or decree. An agency also must provide the court with the administrative record, a summary of the public comments, and access to the record of any public hearings on the proposed decree or agreement. The Department of Justice, or the agency litigating a matter independently, must certify to the court its approval of certain terms included in an agreement or decree, including terms that convert a discretionary authority into a nondiscretionary duty. A court may not approve a consent decree or settlement agreement unless the agency has sufficient time and procedures to comply with federal administrative procedures, other rulemaking statutes, and applicable executive orders. Finally, courts must review a consent decree or settlement if an agency files a motion to modify the decree or agreement on the basis of changed facts or circumstances.