And quite a party it's shaping up to be, too.
Rumour has it, Kate's enlisted the help of Prince William's close pal, club owner Guy Pelly, to plan the secret bash, where her hubby and his pas will be able let their hair down for a right royal knees-up.
It's thought Katehas set her heart on holding the secret party at one of the royal residences (rumoured to be Kensington Palace or Highgrove House) with privacy her main concern.
While it's just not practical for the royal party to celebrate at a night spot in London, Kate has reportedly tasked William's pal Guy Pelly with re-creating one of the Prince's favourite clubs inside their chosen palace.
And with exclusive haunts Whisky Mist, Boujis and Mahiki among his favourites, it sounds like it could well be a party to remember!
What do you think Kate Middleton has in store for Prince William's birthday bash? Let us know in the comments box below…