Siriano revealed on Twitter that he and Walsh had given each other commemorative "engagement bracelets".
"Yesterday @BradWalsh and I exchanged engagement bracelets," the designer tweeted on Sunday (July 28).
"Thank you everyone for the many well wishes and kind words! It's been overwhelming!"
His fiancé Walsh replied: "We've told our mums, so now we can tell you. Yesterday we exchanged engagement bracelets.
"We are happy and we are happy that you're happy and that's all that matters!"
The couple have been together for six years.
Siriano famously competed in Project Runway's fourth season, and became the show's youngest winner.
He currently runs his own successful fashion line.
Walsh works as a singer-songwriter and producer, and has also had his photography featured in publications such as the New York Times and Elle.
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