The win will move the Argentine into the top 10 when the rankings are released Monday.
Monaco was in his second final in two weeks. He lost to Janko Tipsarevic in Stuttgart one week ago. He has six career titles, all on clay.
The 34-year-old Haas was bidding to win his second title at home. Last month, Haas beat Roger Federer to win in Halle on clay.
The Hamburg native started the year ranked outside the top 200 following hip and elbow surgery in 2010. Having started the week at No. 49, Haas is projected to climb to around No. 35.