The suspects - four Russians, two Ukrainians and two Spaniards - made major investments in real estate on the coast of Catalonia in northeastern Spain, police said.
The authorities seized 191 properties and 142 financial assets with a total estimated value of €62 million which they suspect were acquired with money from Russian organised crime.
The suspects were arrested in Reus and the coastal towns of Cambrils and Salou in the province of Tarragona, some 100 kilometres (60 miles) south of Barcelona.
Before the collapse of the ruble in 2014 many Russians bought real estate in Spain, taking advantage of a sharp drop in prices that followed the collapse of a decade-long property boom in 2008.
The bulk of the investments were made in Catalonia and the southwestern region of Andalusia.