Reciprocity Ensures Streamlined Use of Lifesaving Treatments Act of 2021 This bill establishes a reciprocal marketing approval process that allows for the sale of a drug, biological product, or medical device that has not been approved by the Food and Drug Administration (FDA) if the product is approved for sale in another country and there is an unmet need. Specifically, the bill requires the product's sponsor to demonstrate, among other things, that (1) the product has been approved in one of the specified countries, (2) neither the FDA nor any of the specified countries have withdrawn approval for the product because of safety or effectiveness concerns, and (3) there is a public health or unmet medical need for the product. The FDA may decline approval if the FDA determines that the product is not safe or effective. The FDA may condition reciprocal approval on the conduct of postmarket studies. The FDA must issue a decision on whether to grant a request for reciprocal marketing approval within 30 days of receiving the request. Congress may pass a joint resolution to grant reciprocal marketing approval of a product that the FDA declines to approve through the reciprocal process.