"The Duke is one of more than 2000 crisis volunteers who are trained to support anyone, anytime, whatever their crisis may be," Kensington Palace said in a statement on Saturday to mark Volunteers' Week.
Prince William also revealed his involvement during a Zoom call with volunteers last month, telling them: "I'm going to share a little secret with you guys, I'm actually on the platform volunteering".
The Duke and Duchess of Cambridge have been keeping busy during the pandemic, conducting official royal duties online and video calling with various charities and organisations.
To celebrate Volunteers' Week, they called two organisations – Conscious Youth and Machynlleth Community Corona Response – to thank them for their service.
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"It's National Volunteering Week and I want to say a big thank you from both of us," Prince William said on one call.
"Thank you for all the volunteering you're doing, thank you for all the time and effort you're putting in. It's been hugely rewarding and important that you guys are doing that and you have been a lifeline to all the people who you've helped in the area."
Kate added that everyone had "something to give back".
"One of the things that would be amazing is if everyone in their communities was to carry on and still celebrate volunteering in a way that they have been during the pandemic," she said.
The Duchess of Cambridge has herself been volunteering by calling and checking in on people who are self-isolating or vulnerable.
Meanwhile, she and Prince William have reportedly launched legal action over a magazine article published in the high-society magazine Tatler last month.
The magazine – which is known for reporting on the private lives of British celebrities, socialites and royal family members – claimed in the article that the Duchess felt "exhausted and trapped" in the wake of Megxit.
It quoted a close friend who said Kate was "furious" after Prince Harry and Meghan decided to step down as senior members of the royal family, leaving others to pick up extra work.
Kensington Palace has dismissed the article as full of "inaccuracies" and "misrepresentations".
"This story contains a swath of inaccuracies and false misrepresentations which were not put to Kensington Palace prior to publication," it said in a statement.
