"So...can you guess what my big announcement is?" she captioned a photo of herself smiling and pointing to a large diamond ring on her left hand. "Yes, my amazing boyfriend Scott and I are engaged!"
"This is the guy that Valentin Chmerkovskiy did a shout-out to on Dancing With the Stars!" she added, referring to the season 18 episode when Chmerkovskiy thanked McKellar's "amazing boyfriend Scott" for being so cool about letting him dance with the actress.
"Hugs to you all!" she tweeted at the end of her announcement. On Facebook, she added: "I'm wishing all of you the happiness and joy that I feel right now!"
This will be the second marriage for the Wonder Years alum, who is also a brilliant mathematician and the author of four non-fiction books. She was previously wed from 2009 to 2012 to composer Mike Verta, with whom she has one son, Draco.