Ms Grisham replaced Sarah Sanders, who left the position last week after two and a half years.
Mrs Trump made the announcement herself on Twitter Tuesday.
“I am pleased to announce @StephGrisham45 will be the next @PressSec & Comms Director!” the First Lady tweeted. “She has been with us since 2015 – @potus & I can think of no better person to serve the Administration & our country. Excited to have Stephanie working for both sides of the @WhiteHouse.”
Ms Grisham will also serve as the White House communications director. Mrs Trump described as “terrific” the possibility of choosing Ms Grisham as the White House press secretary in a recent media interview.
Ms Sanders welcomed the announcement of her replacement on Twitter, saying Ms Grisham “will be an incredible asset to the president and the country.”
Ms Grisham would become the 30th White House press secretary when she her appointment commences effective July 1.