Roma revealed that Marseille had agreed to pay 25 million euros ($29.3m) for Strootman, with a potential three million euros more to come in performance-related add-ons.
The 28-year-old is Marseille’s second signing of the summer transfer window, following the arrival of Croatian international defender Duje Caleta-Car from Red Bull Salzburg in Austria.
Strootman played under current Marseille coach Rudi Garcia when he joined Roma from PSV Eindhoven in 2013.
He has won 41 caps for the Netherlands, although two serious knee injuries marred his time at Roma and led to him missing the 2014 World Cup.
The signing of Strootman will help Marseille fill a void left by the departure of Cameroon international Andre-Frank Zambo Anguissa, who was sold to Fulham earlier this month for 30 million euros, an offer the club deemed too good to refuse.
Would you like to place a soccer bet? Click here to get started in 3 easy steps.