Regulatory Certainty Act of 2018 This bill amends the Federal Water Pollution Control Act (commonly known as the Clean Water Act) to specify that the Environmental Protection Agency (EPA) must determine, in consultation with the U.S. Army Corps of Engineers, that a discharge of dredged or fill materials into a disposal area will have certain unacceptable adverse effects before the EPA prohibits the specification, or restricts the use, of an area as a disposal site. The period during which the EPA may make such a prohibition is limited to the period that: (1) begins on the date that the Corps of Engineers provides notice to the EPA that the Corps of Engineers has completed all procedures for processing an application for a permit for dredged or fill material and is ready to determine whether the permit should be issued, and (2) ends on the date that the Corps of Engineers issues the permit. The Corps of Engineers must ensure that the period consists of at least 30 consecutive days. The Corps of Engineers may issue a permit for dredged or fill material only after the Corps of Engineers provides notice to the EPA.