Colonia Infrastructure Improvement Act of 2021 This bill provides support for low-income communities with economic hardships that are commonly referred to as colonias and are located along the United States-Mexico border. Specifically, the bill reauthorizes through FY2026 and revises grant programs to provide assistance to those communities to construct drinking water infrastructure and wastewater infrastructure. Both grant programs are administered by the Environmental Protection Agency. In addition, the bill directs the Department of Transportation to establish a grant program to assist projects that facilitate the state of good repair of surface infrastructure in, or for the benefit of, colonias.