During a meeting with staff at a mental health charity yesterday, the 35-year-old revealed that Prince Harry hasn't popped the question to him.
'He hasn't asked me yet, just to clear that up. It could be a sensitive subject', quipped Wills.
Prince Harry was by his older brother's side when he tied the knot at Westminster Abbey back in 2011, so fans have been hopeful that Kate Middleton's husband would return the favour.
There might be another reason as to why Harry's yet to ask William to step up - it could breech royal protocol.
'The best man or best men are officially called supporters', Marlene Koenig, a British and European royalty expert, recently told Town & Country. 'William will probably be Harry's supporter.'
It is likely, however, that Prince George and Prince Charlotte will nab roles in the ceremony.
It's likely George and Charlotte will have roles in the ceremony, like they did at Pippa's wedding last year
While filming a video for CALM: the Campaign Against Living Miserably, the dad-of-two also opened up about the strong bond he shares with his younger sibling.
'Our relationship is closer than it's been because of the situation we've been through', he said.
'Losing our mother at a young age, it's helped us travel through that difficult patch together. You're like-minded. You go through similar things, it's a bond and it's something you know you've tackled together and come out better for it.
Prince Harry was William's best man in 2011
'That's the thing about being a best mate, inevitable one of you is sometimes on an up while the other is on a down. You're always there for each other and repaying that favour.'
Prince Harry, 33, is set to wed American actress Meghan Markle on Saturday 19 May 2018.
The handsome redhead asked for the 36-year-old Suits star's hand in marriage during a quiet night in at his apartment in Kensington Palace.
Harry romantically proposed with a ring that he designed himself. The stunning sparkler features diamonds belonging to his late mother Princess Diana and a glittering stone from Botswana.
The loved-up pair - who dated for around 16 months before Harry got down on one knee - will become man and wife at St George's Chapel at Windsor Castle.