"I can think of no better person to serve the Administration & our country," Trump said in a statement posted on Twitter.
The first lady said Grisham will also serve as White House director of communications, a position that's been vacant since former Fox News executive Bill Shine left the role in March.
Grisham will replace the current press secretary, Sarah Huckabee Sanders. President Donald Trump announced two weeks ago that Sanders, plans to step down at the end of June.
"(Grisham) will be an incredible asset to the President and the country," Sanders said in a statement posted on Twitter. "I’m sad to leave the WH, but so happy our team will be in such great hands. Stephanie will do a phenomenal job."
Axios reported President Trump wanted Grisham in the position and that he's said he likes and trusts her. The news site noted she's one of the few officials who has been with President Trump since his campaign.
She will continue to serve as the first lady's spokeswoman as well, CNN reported.
Grisham will become the fourth woman to serve as White House press secretary.
Before serving as the first lady's spokeswoman, Grisham worked under Trump's first press secretary, Sean Spicer, The Washington Post reported. She also previously worked on Republican Mitt Romney's 2012 presidential campaign, according to the newspaper.
"During the campaign, she developed a good relationship with the president, and that's carried through," Sanders said of Grisham in an interview late last year, according to the Post. "She has developed a great amount of trust from both the president and the first lady, which is a pretty high commodity here. There aren't a lot of people who have a lot of regular interaction with both of them."