Ryan, 40, took to Instagram to make the announcement as well as shared a sweet poem from her younger sibling and bride-to-be Meredith.
"I have a role in my sister's wedding... Just shed a tear," Seacrest wrote.
"You've been by my side all of my life, and I wouldn't want you anywhere else, when I become a wife. "I'm so lucky to have you as my brother, and couldn't imagine celebrating with any other," read the creative proposal.
"So basically what I'm trying to say... Will you be my Man of Honour on my special day?!"
Ryan is currently filming the 15th and final season of "American Idol", and continues to executive produce "Keeping Up with the Kardashians" and "Shahs of Sunset".