Boasson Hagen (Dimension Data) claimed the closing day's honours in a sprint at the end of a challenging 197.8km stage from Bornem to Grammont.
"The race was hard but that suited me well," Boasson Hagen said. "I was feeling strong but the wet made the race difficult so you had to stay focused.
"We got away with a very strong group and it was important to not waste energy. I came to Eneco with the goal of possibly winning a stage and the overall but the overall didn't go to plan. So I was really motivated to win today's stage.
"I knew Terpstra would be going for the overall so I tried to ride clever, using that to my advantage but Naesen was there and he was very strong. In the finale I tried to save all I could for the last 200m. I am happy it was enough to win the stage."
Terpstra (Etixx-QuickStep) was placed fifth going into the stage, almost half a minute behind Australias Rohan Dennis (BMC), the overall leader.
But Dennis's hopes of victory were shattered when he crashed with less than 50km
"The plan was to attack to put BMC and Rohan Dennis in difficulty, and it worked perfectly," Terpstra said. “I’m extremely happy and proud to finish off this great job of the team, who was incredible today.
"We were aware of the fact that we had several cards to play in the general classification at the start of the day and we were keen to make use of these numbers."
"The Muur van Geraardsbergen has a special place in cycling and it means a lot for me to seal the overall victory here.
For Terpstra this was the perfect preparation for his main end-of-season target, the world championship team time-trial in Doha.
Finishing third in the final stage was Oliver Naesen (IAM), who finished second overall.