The move will bolster Al Hilal's defence, with Kalidou Koulibaly still away at the Africa Cup of Nations with Senegal. Mari played 16 games with Fiorentina in all competitions this season.
"Al-Hilal have signed a contract with Spanish defender Pablo Mari to join Al-Hilal from Italian club Fiorentina on a six-month contract, with an option to extend for another year," the Saudi club said in a statement.
The 32-year-old defender won the FA Cup with Arsenal in 2020 and lifted the Copa Libertadores with Brazil's Flamengo in 2019.