"There will be no on-the-job training for Kirstjen", Trump said.
Also considered for the job was House Homeland Security Chairman Michael McCaul of Texas.
Nielsen previously worked as chief of staff to John Kelly - Trump's current White House chief of staff -when Kelly served as Trump's first DHS secretary. "Mr. President, if confirmed, it will be the highest honor of my life to again work with and support these remarkable public servants and to continue to serve all of the American people".
On Wednesday, she greeted Canadian Prime Minister Justin Trudeau and his wife Sophie Grégoire Trudeau to the White House in a jaw-dropping grey pinstripe pantsuit outfit. Earlier in her career, she worked as a special assistant to former President George W. Bush.
Nielsen is now the White House principal deputy chief of staff.
Her nomination must now be confirmed by a vote in the Senate. Elaine Duke has been serving as acting DHS secretary since then.
Mr. Kelly and Ms. Nielsen arrived at the White House after a tumultuous period that led to the departure of Reince Priebus, Mr. Trump's first chief of staff, and Sean Spicer, his original press secretary. "It seems like more of a lazy pick than a wide search".
Her nomination comes at a time when the president and the Federal Emergency Management Agency, which DHS oversees, have come under staunch criticism for the response to Hurricane Maria in Puerto Rico.
Former DHS Secretary Tom Ridge - the first person to ever hold the position, which President George W. Bush created in the aftermath of the September 11 terrorist attacks - praised Nielsen in a statement, calling her a "homeland security veteran" who is "extremely well versed" in the threats facing the U.S.