The Inglourious Basterds star has been close friends with the style guru, who is creative director of the Hugo Boss womenswear label, for years and she frequently wears his designs and supports him at fashion shows.
Their friendship reached new heights on Friday when Diane flew to Tulum in Mexico to see Jason, 33, tie the knot with his longtime partner Gustavo, who is chief financial officer of the Jason Wu brand.
She posted a picture of herself with the happy couple on Instagram and wrote in the caption,"Congratulations.... I love you so much guys @jasonwu @tavonyc...." and in a lengthy hashtag, she added, "...my happiness for my friends is too much to sit still."
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Diane also uploaded a video of Jason and Gustavo kissing beside their wedding cake and added, "Love always" in French.
Jason met his partner on New Year's Eve in 2005. The designer is best known for creating the dresses worn by U.S. First Lady Michelle Obama at both Presidential inaugurations.
Diane previously told V Magazine in 2015 she met Jason when she requested one of his gowns for the Cannes Film Festival years ago and he delivered it himself.
"Jason is a wonderful designer and a truly good and fun person," she said. "He's generous to a fault, and if I could, I'd fold him up and put him in my suitcase wherever I go!"
The 39-year-old German has been dating actor Joshua Jackson for ten years but she is in no hurry to follow in Jason's footsteps.
"I can tell you why we're not married: We're not religious," Joshua explained Glamour magazine in 2014. "I don't feel any more or less committed to Diane for not having stood in front of a priest and had a giant party."
The actress was previously married to French director Guillaume Canet for four years until 2005.
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