The couple, who met at the Democratic National Convention in 2000, have been married four years. Roos is a political consultant, E! Online reported.
A legal separation divides a couple's assets, but allows for a smoothed reconciliation should they decide to get back together again.
"He doesn't know what the future holds, but he does care deeply for her," Staci Wolfe, Arnold's spokeswoman, told People magazine.
Arnold's attorney is asking that the actor's Beverly Hills home and premarital earnings be excluded from their joint property even though there was no prenuptial agreement. In California, property is normally split evenly in a divorce.
Arnold was married to comedian Roseanne Barr for four years and then to makeup artist-actress Julie Champnella for another four years, both in the 1990s.