The new father will start the annual 135-mile Ring O'Fire race that challenges competitors to circumnavigate the Isle of Anglesey in north Wales.
"His Royal Highness will meet a number of the runners and their families and a selection of volunteers involved in the administration of the race before officially starting the Ultra Marathon," Kensington Palace said ahead of the engagement.
The tough race follows the Anglesey Coastal Path around the island which William and Kate have called home since 2009.
It is also the place where their one-month-old son Prince George has spent the last couple of weeks since travelling there with his mother Kate Middleton from his grandparents' house in Berkshire.
Since being back on Anglesey, Kate has been getting back into the swing of relaxed island life. She was spotted stocking up on food at a supermarket in Anglesey on Monday.
Casually dressed in a striped jumper, black leggings and flat shoes, with her hair pulled back into a ponytail, the 31-year-old pushed her trolley around the aisles picking up supplies for herself, her husband and their baby boy.
The new father's public appearance at the race launch will most likely be one of his last on the island, before his tour of duty ends in September.
When he opened the Anglesey show on 15 August, William spoke of his love for the island and its residents.
"I know that I speak for Catherine when I say I have never in my life known somewhere as beautiful and as welcoming as Anglesey. The views across the Menai straits are undoubtedly among the most stunning in the British Isles.
"I know that both of us will miss it terribly when my search and rescue tour of duty comes to an end next month and we have to move elsewhere.
"From the bottom of my heart, thank you for making my wife and me so welcome when we arrived here, as you do thousands of visitors each year.
"This island had been our first home together, and it will always be an immensely special place for us both.
"Catherine and I look forward to returning again and again over the coming years with our family."