The National Lottery windfall has been won with a ticket bought somewhere in Denbighshire for the 1 April draw.
It matched the Lotto Millionaire Raffle draw - AQUA 6323 7030 - but no-one has come forward.
The mystery ticket holder has until the end of September to submit their claim - otherwise the money will go to lottery good causes.
"We're urging everyone who bought a Lotto ticket in this area to check their old tickets again or look anywhere a missing ticket could be hiding," said Andy Carter from the National Lottery.
"Try checking in the pockets of clothing, in wallets, bags and down the back of the sofa - someone out there could literally be sitting on a fortune."
The search is on for the winner of an unclaimed winning lottery ticket worth £1m.