Pasalic moved to Stamford Bridge in 2014 and has had temporary spells across Europe, including a year with Italian giants AC Milan.
He spent the 2017-18 campaign with Spartak Moscow, making 21 appearances in the Premier League, scoring four goals.
A second stint in Serie A beckons after Atalanta added him to their squad for the 2018-19 campaign, with the Italian club holding an option to make the move permanent.
"Mario Pasalic will spend the upcoming season on loan at Serie A side Atalanta," Chelsea confirmed in a statement.
Mario Pasalic will spend the upcoming season on loan at Serie A side Atalanta.— Chelsea FC (@ChelseaFC) July 25, 2018
Good luck, Mario! https://t.co/gSOqsKny5q
"The midfielder has previous experience in Serie A after being loaned to AC Milan in 2016/17.
"Having played the second half of our friendly win over Perth Glory on Monday, he now heads back to the Italian top flight to continue his development with Atalanta."