Interregional Transmission Planning Improvement Act of 2021 This bill requires the Federal Energy Regulatory Commission to issue a rule to evaluate the effectiveness of planning processes for projects concerning electric energy transmission across regions. The rule must address (1) the effectiveness of existing planning processes for identifying interregional transmission projects that provide economic, reliability, operational, and public policy benefits, including reductions in carbon emissions; (2) changes to such processes to ensure that efficient, cost-effective, and broadly beneficial transmission solutions are selected for cost allocation; and (3) cost allocation methodologies that reflect the multiple benefits provided by interregional solutions.