The winning ticket was bought in Merthyr Tydfil or Rhondda Cynon Taff for a one-off Team GB-inspired Lotto Medal Draw on 27 August 2016.
It was one of 27 millionaires created to celebrate the number of gold medals won at the Rio Olympics.
If it is not claimed, the cash will go to charity.
Camelot said it has a code of RUBY 6863 3083.
There was another £1m ticket purchased in the area - in the Euromillions UK Millionaire Maker draw which has four winners weekly, two on Tuesday and two on Friday.
That one was successful in the 3 February draw and had a winning code of ZXPR 17675. The ticket's owner has until 2 August to come forward.
"Try checking in the pockets of clothing, in wallets, bags and down the back of the sofa. We have the champagne on ice and our fingers crossed that the lucky winner comes forward to claim their winnings," said the National Lottery's Andy Carter.
A £1m prize winner will lose their cash unless they come forward by midnight on Thursday, lottery operator Camelot has said.