Pam Bondi is described as a long-standing and close ally of Trump.
She was one of Trump's lawyers when he was first impeached – and acquitted – after being accused of pressuring Ukraine's government to achieve personal political gain.
"For too long, the partisan Justice Department has been used as a weapon against me and other Republicans – no longer", Trump writes on social media, adding that Bondi will focus on "making America safe again".
Bondi, from Tampa, has more than 18 years of experience as a prosecutor and has previously held the role of Florida's highest legal office (Attorney General).
Gaetz's resignation on Thursday came a day after he met with leading Republican senators in Washington. It is the Senate that approves a Justice Minister.
On Wednesday, the House Ethics Committee also voted not to release a report on Gaetz and his alleged sex purchases. The committee's investigation was discontinued in connection with Gaetz leaving his seat in the House of Representatives after Trump's nomination.
