Impaired Waters Improvement Act - Authorizes the Administrator of the Environmental Protection Agency (EPA) to make grants to reduce the discharges of nitrogen, phosphorus, or sediment (pollutants) in an area that is subject to a total maximum daily load (TMDL) for those pollutants under the Federal Water Pollution Control Act (commonly known as the Clean Water Act). Makes eligible for grants the owners and operators of: (1) publicly owned treatment works or storm water management practices serving areas subject to the TMDLs, or (2) privately owned farms implementing methods to reduce discharges of those pollutants in those areas.

Directs the Administrator to increase by 5% the civil penalty for violations of the Clean Water Act. Establishes the Impaired Waters Improvement Fund for making expenditures to carry out the grant program. Transfers for each of FY2015-FY2019 the amount attributable to the penalty increase or $100 million, whichever is less, to the Fund.