Arn's 6-3 6-3 victory in the final over the second seeded Belgian gave her a second WTA Tour title, after Estoril in 2007 when she was a qualifier.
But while she didn't have to face a seeded player in Portugal, she upset four seeds this week, including the No 1 seed, Russian Maria Sharapova.
Against world No 23 Wickmayer, she put in a steady performance, not having a break point against her in the opening set, and was helped by plenty of unforced errors from her opponent.
She also mixed up her game, taking the set with the one-two combo of a drop shot that drew Wickmayer into the net and then a delicate lob for the winner.
A solitary break in the second set proved to be sufficient, although Arn stuttered in closing out the contest.
Wickmayer saved one match point on her own serve to take the score to 5-3.
Arn then failed to convert her second match point by double faulting, before finally taking advantage of her next opportunity.
She admitted that, as in her final game against Sharapova, she began being hit by nerves with the finish so tantalisingly close.
"Yes, especially when I had the match point and I did a double fault, that was shaking," she said.
"I told myself just hit it. If it's in, it's in, if it's out, it's going to be out anyway, so just hit it."
When she end did come, she fell on to the court, overcome.
It was, she said, "my dream come true".
"The feeling, I couldn't believe it, I cannot describe it," she said.
"That's what you've wanted and you've got it."
Wickmayer had three three-setter this week, including a marathan 2hr 52min against China's Shuai Peng yesterday.
She agreed she might have been fatigued, mentally rather than physically, heading into the final.
"I had to to come back a lot of times and I had to fight though a lot of difficult moments," she said of her previous rounds.
"It makes it not easy to stay fit mentally."
She was less disappointed with the defeat that with her performance, but was overall pleased with how the week had done.
"I had one bad day, but I had four really good ones , I beat four good players," she said.
"I mean, shit happens, and I guess today was not my day."
