The stamps show William and Kate sharing a hug, the Daily Express reported.
The official engagement portraits that feature on the stamps were taken by photographer Mario Testino.
One stamp having a "casual image" costs 46 pence, while the more formal image stamp costs 1.10 pounds and is for overseas use.
William has featured on stamps twice before, one marked the Queen Mother's 100th birthday and the other marked his own 21st birthday.
Post offices will sell these stamps from April 21 - the queen's birthday - but they can also be ordered via the Royal Mail's website from April 7.
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