Regional Greenhouse Gas Reduction Act of 2021 This bill establishes an Office of Regional Greenhouse Gas Reduction Programs within the Environmental Protection Agency (EPA), a grant program to support regional greenhouse gas reduction programs, and an interagency task force. The office must support existing regional greenhouse gas reduction programs and facilitate the establishment of new regional greenhouse gas reduction programs. Such programs use market-based tools to reduce greenhouse gases across states at the regional level. In addition, the office must establish a program that awards grants to a state, local government, or regional consortium of such governments to take preliminary steps towards developing or participating in a regional greenhouse gas reduction program. Finally, the EPA must establish an interagency task force to support new and existing regional programs.