The pair confirmed the good news exclusively to People magazine earlier today.
Derringer, 49, who most recently starred on "General Hospital," proposed to the "DWTS" judge this past Saturday at the same place they had their first date -- a secluded cove along the California coast.
"Saturdays are special to us, because our first date was on a Saturday and we have celebrated with a toast, a visit to a beach, special date, a motorcycle adventure, or a road trip on every Saturday since," Inaba, 48, told the magazine. "So on Saturday, we returned to the place of our first date and got engaged at the same place we shared our spontaneous and magical first date."
Derringer's proposal started with a letter to Inaba, "expressing how much she meant to me, how much she had changed me, how much she had taught me in our time together thus far and what my hopes and dreams and intentions were for our future together," he said.
"I then got down on one knee under what I will always remember as the most serene and tranquil night sky I’ve ever seen and asked my beautiful girl if she would do me the honor of being my wife, to which she thankfully replied yes," he added.
The couple first went public with their romance earlier this spring. They later moved in together with a menagerie of rescue pets, including three dogs, a cat, a raven and a fish named Steve.
"I knew after our first romantic date, that if she felt for me what I felt for her, then it was something that was going to be significant and life changing and lasting," Derringer told People. "There was just a powerful and palpable sense of destiny that I could not deny."
For her part, Inaba remembered, "Everything about the evening felt so ‘right.’ There was an ease and an authentic familiarity. We talked about everything under the stars ... even marriage. There was a strong sense of destiny that night, and I think I knew then, deep in my heart, that he and I were going to be together forever."
For Inaba, who was previously engaged to accountant Jesse Sloan, this will be her first marriage.