Cavill, 33, and King, 19, first met at London club Mahiki last year. They were spotted together at a rugby match at Twickenham.
He later brought her to the London premiere of his movie "Batman v Superman: Dawn of Justice". They also walked the red carpet together at Vanity Fair's Oscar after-party last month.
The Superman depicter once said the large age gap between them had never been a big deal, reported Aceshowbiz.
"People say age is just a number," he had said. "It's actually real and true sign of someone's maturity. But in this case, she's fantastic.