The 31-year-old moved to Craven Cottage from Krasnodar last summer and went on to make 18 appearances for the club across all competitions.
However, he has now sealed a return to Russia after just one year in England, having completed a loan move to Rubin.
"The club can confirm that Ragnar Sigurdsson has joined Rubin Kazan on a season-long loan deal," read a statement on Fulham's official website.
"The centre-back scored once in 18 appearances for the Whites last term. We wish Ragnar the best of luck during his time in Russia."
Fulham kick off their Championship campaign at home to Norwich City on Saturday.