Thérèse Coffey will replace Amber Rudd as work and pensions secretary after Rudd resigned on Saturday, Downing Street has confirmed.
Coffey has served as an environment minister since 2016. She is the Conservative MP for Suffolk Coastal.
Rudd resigned on Saturday after Johnson withdrew the whip from Conservative MPs who voted against the government.
Rudd called the sacking of the MPs an "assault on decency and democracy" and said it "has stripped the Party of broad-minded and dedicated Conservative MPs."
Rudd congratulated Coffey, tweeting: "I know she will do an excellent job in a first rate department."
Coffey has been an MP since 2010 and has a PhD in chemistry. She backed Remain in the 2016 Brexit referendum.
SNP parliamentary leader Ian Blackford criticised the appointment, tweeting that Coffey is "well known for her attack on pensioners."
"Yet another appointment that shows the UK government being full of extreme right wingers determined not only to deliver a no deal Brexit but attack pensioners rights," he said.