The 35-year-old singer claims she's frequently mixed-up with the Oscar-nominated actress, especially when it comes to invoices.
"So I get this phone call one day, and it was about this crazy phone bill. It was really high..." the 35-year-old told Amazon's Style Code Live.
"I've had a $10,000 phone bill, too. And it's because, travelling overseas, I forgot to do the international package, and all of that stuff. So when I saw it (Michelle's phone bill), and I saw where the phone calls were to, I said, 'Oh no! That's not me, that's the other Michelle Williams'," she recalled.
The pricey phone bill wasn't the only thing the singer received that wasn't meant for her - she also got a call about ordering a very expensive children's playhouse.
"I got another phone call, 'Did you buy this playhouse?' And it was like, in the thousands of dollars. I was like, if I have kids, they'll get the $400 playhouse! I'm not buying all this crazy stuff!" she joked.
"So, I was like, 'That's for Matilda (Michelle Williams' daughter with the late Heath Ledger), not me...'"
To make matters worse, the stars are both the same age, but singer Michelle insists she doesn't mind sharing her name with the former Dawson's Creek star, because she really admires her career.
"I absolutely adore Michelle," she gushed. "I think her acting, everything, all the unique things she does... she's been conquering Broadway too."