The MP was named as a Defra minister in May following the Conservative victory in the general election. He replaces deposed Liberal Democrat MP Dan Rogerson, who lost his seat in the election, as water minister, a role he held for two years between 2013 and 2015.
His responsibilities will include the natural environment; floods and water; resource and environmental management; and rural affairs.
He will be shadowed by Labour MP Barry Gardiner who yesterday confirmed to Utility Week that he will replace Angela Smith as the water minister within the Labour shadow cabinet. Gardiner has significant environmental policy experience and will also be running for the position of Environmental Audit Committee chair.
Stewart will deputise for Environment secretary Liz Truss on the Environment Council and will have lead responsibility within the department for the Environment Agency, Natural England, and the Forestry Commission.
Stewart has previously chaired the defence committee and been a member of the National Security Strategy joint committee. He has been vocal in the past about support for farmers and for upland reform.
George Eustice will continue in his role as farming minister.