The 32-year-old led in Morocco at the end of every round, and suffered no nerves on day four as he held off the threat of Mikko Ilonen and David Horsey, who tied for second place.
"I’m super happy; really proud that I stayed that calm," said Siem. "There was a lot of pressure – Mikko had a great start again – so I’m really happy that I scrambled well and came back nicely.
"I’ve never been in that situation so I spoke to Bernhard [Langer] last night to see if he could give me some advice on how to cope with being four shots in the lead, so thanks Bernhard!”
Siem needs to be in the world's top 50 if he is to be invited to the US Masters in Augusta - whether he finds himself there when the new rankings are issued depends on how others get on at the Shell Houston Open on the US Tour.
“That will be unbelievable,” Siem said when asked what it would mean to qualify for the event.
“It’s been my dream since I was a kid so if that comes true it will be unbelievable. It would be wonderful to play the Masters.
“I just want to be on that ground. I’m not going to cry but it would be very emotional.”