Platform Competition and Opportunity Act of 2021 This bill generally prohibits operators of covered platforms from acquiring the stock or other share capital or the assets of another person engaged in commerce or in any activity affecting commerce. Covered platforms are online platforms that (1) have at least 50 million U.S.-based monthly active users or at least 100,000 U.S.-based monthly active business users, (2) are owned or controlled by a person with net annual sales or a market capitalization greater than $600 billion, and (3) are critical trading partners for the sale or provision of any product or service offered on or directly related to the platform. The Federal Trade Commission or the Department of Justice must designate whether an entity is a covered platform, and both must carry out enforcement activities. The bill provides for any person (other than a foreign state and any instrumentality thereof) who is injured by an activity forbidden under the bill to recover triple damages. (A foreign state may only recover actual damages.) Further, the bill specifies the U.S. Court of Appeals for the District of Columbia Circuit as the judicial venue for a covered platform's appeals related to (1) the designation as a covered platform, (2) other administrative and enforcement proceedings, or (3) a final order issued in any district court.