On Tuesday the actress was seen wearing a diamond engagement ring on the set of her appropriately-named new film, The Wedding Ringer.
"It's only been three months but she knows Ryan is the one," a source told US Weekly. "When you know, you know."
Ryan, 26, is said to have popped the question on Wednesday, days after the happy couple made their red carpet debut at the Emmy Awards.
His new fiancee, 27, hinted at her joy when she tweeted soon after, "Flying high, I could literally touch the sky with you…."
She posed for an Instagram picture with her mum Ashley and sister Briana with the caption, "A sweet celebration w my sweet soul sisters."
The smitten star then shared a photo of two penguins walking side-by-side with a note that read, "I love u @ryansweething :)"
"(She's) never felt this way. She knows this is it. And the ring is gorgeous," a friend told US Weekly.
Kaley and Bahamas-born Ryan have been inseparable since August.
"A million people, all I see is her," Ryan wrote next to a romantic photograph of Kaley, who he calls "the most beautiful girl".
Kaley, like her husband-to-be, was once a nationally-ranked amateur tennis player, but she retired when she was 16. The sporty actress is now an avid equestrian and owns four horses.
Prior to meeting Ryan, she had a brief romance with Man of Steel's Henry Cavill, and she secretly dated her Big Bang Theory co-star Johnny Galecki for two years.