The mom-of-three took to Instagram with an emotional video which left fans in disbelief.
In the clip, Jennifer celebrated her 50th birthday a few days early and her gift was amazing.
WATCH: Jennifer Garner sparks engagement rumors with huge ring
As the Yes Day actress enjoyed lunch at a plush-looking restaurant, she was surprised by none-other-than, legendary performer, Donnie Osmond.
Jennifer appeared close to tears as he sat next to her and began to sing.
She explained the scene in her caption which read: "My first (pre)birthday surprise was a doozy — the one and only, legend in his own time, object of my childhood adoration and devotion — @donnyosmond showed up to knock my purple socks off.
Jennifer was stunned by her birthday surprise!
"He took an hour and a half of his day to sit with me, sing my favorites, and to give me a master class on how to be a class act."
Her fans were almost as blown away by the moment as Jennifer was and they commented: "I don't believe it. He is everything," and, "I'm dead, living vicariously through you".
There were shocked face emojis and plenty of love hearts too as Jennifer's social media followers also wished her happy birthday.
Jennifer shares three children with her ex Ben Affleck
Her celebration comes just a day after she broke her silence following Ben and Jennifer's engagement news.
She shared a moving quote alongside a sun-kissed photo of herself relaxing on a yacht.
The Alias star was pictured modeling a black sundress, oversized shades and windswept hair as she smiled at the camera.
The gorgeous snapshot was accompanied by the message: "Wisdom from Mom – happiness is your own responsibility. The life you're living while you wait for the grade, the college, the job, the man, is your actual life. Decide to enjoy it and work towards your own peace and contentment."
JLo confirmed the news of her engagement to Ben on 8 April, not long after he popped the question with a dazzling ring worth an estimated $10million.
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