The hitmaker told her fans on Instagram what had happened as she shared a string of images from her sunny break in Ibiza.
However, it seems the star had the last laugh, as she admitted that having some time without her phone had actually been "bliss".
Sharing a photo of her wearing sunglasses, the 'Shout Out To My Ex' singer wrote: "Shout out to whoever stole my phone.
"Not having one for a little while was truly bliss."
Thirlwall, 29, then uploaded holiday snaps showing her on a plane, enjoying sunset on a beach and relaxing by the pool and in restaurants with friends.
"Anyways here’s me being a basic b**** in Ibiza a cutla weeks ago," she said.
She also shared a rare snap of her and boyfriend Jordan Stephens, who the star has been dating for more than two years.
The post was Thirlwall's first in two weeks and fans said they had been concerned.
"I'm so glad you're OK, got worried," one person posted in the comments section.
Another said: "OMG I hope you were safe in that phone event."
"I was beginning to worry as you hadn’t posted on your Story or anything," said another of the singer's followers.
"Glad you got your phone back!" said someone else.
"I was wondering when you would be back," another person posted.
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