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.