A source confirmed the engagement to E! News, telling the publication: "I've never seen two people so truly in love."
"The way they talk to each other, touch each other. It's incredible."
Sofia, 42, gave fans a glimpse of her happiness as she celebrated Joe's birthday with a tender message on Twitter on Monday.
"Happy happy bday to my love and best friend," she wrote, alongside a sweet photo of the happy couple hugging and smiling. "To the one that makes me laugh like crazy!have a healthy wonderful year babe!!!@joemanganiello."
Engagement speculation was sparked earlier in December when the handsome stars were spotted out jewellery shopping in Los Angeles, where Jo purchased a rose ring for his gorgeous girlfriend.
In July, it was revealed that Sofia and Joe had begun dating, and a month later, the loved-up actress opened up about their relationship. "I'm very happy romantically," she told People magazine. "It couldn't be better!"
Since then, the couple have been spotted on numerous minibreaks and dates, but things seemed to get serious when Joe joined Sofia in Florida and met her family for the first time.