On January 11, 2018, Fitzpatrick announced his decision to forgo his remaining eligibility and enter the 2018 NFL Draft. Fitzpatrick attended the NFL Scouting Combine in Indianapolis and completed the majority of combine drills, but opted to skip the short shuttle and three-cone drill. On March 7, 2018, Fitzpatrick participated at Alabama's pro day, but opted to stand on his combine numbers and only performed the short shuttle, three-cone drill, and positional drills.
The Miami Dolphins selected Fitzpatrick in the first round with the 11th overall pick in the 2018 NFL Draft. Fitzpatrick was the first safety drafted in 2018.
On June 1, 2018, the Miami Dolphins signed Fitzpatrick to a fully guaranteed four-year, $16.4 million contract that includes a signing bonus of $10.04 million.
He made his professional regular season debut in the Dolphins’ season-opener against the Tennessee Titans and recorded six combined tackles and broke up a pass in their 27–20 victory. On September 23, 2018, Fitzpatrick earned his first career start after Jones was declared inactive due to a shoulder injury. He recorded ten combined tackles (two solo) during their 28–20 victory against the Oakland Raiders in Week 3. The following week, he collected ten combined tackles (four solo), two pass deflections, and made his first career interception in the Dolphins’ 38–7 loss at the New England Patriots. Fitzpatrick made his first career interception off Tom Brady in the fourth quarter on a pass intended for Phillip Dorsett, and returned it for a 14-yard gain. During a Week 15 matchup with the Minnesota Vikings, Fitzpatrick intercepted a Kirk Cousins pass, returning it 50 yards for a touchdown. It was his second interception and first pick-six of his career.