Investments in Rural Transit Act of 2021 This bill increases the federal share of operating costs to 80% for projects in certain rural areas with high transit dependency that receive grants under the Formula Grants for Rural Areas program of the Federal Transit Administration. The program provides capital, planning, and operating assistance to states to support public transportation in rural areas with populations of less than 50,000. To qualify for the increased federal share, a project must serve an area that is an area of persistent poverty (a county in which at least 20% of the population has lived in poverty during the most recent 30-year period); a county in which at least 25% of residents are age 65 or older, a county designated as a health professional shortage area on the basis of a primary care or mental health care shortage, or a county with low population density of not more than 20 people per square mile of land area.