Mari has joined the Serie A side after making 22 appearances for the Gunners.
The 28-year-old Spaniard initially joined Arsenal on loan from Brazilian side Flamengo in January 2020 and made the move permanent the following summer.
Wishing you all the best on loan, Pablo 🤝
— Arsenal (@Arsenal) August 11, 2022
Mari spent the second half of last season in Italy on loan at Udinese, where he made 15 appearances.
“Everyone at Arsenal wishes Pablo well in Italy with AC Monza during this next stage of his career,” said a statement on the club website.
“The deal is subject to the completion of regulatory processes.”