The royal "Fab Four," as the in-laws are known, launched Shout, a free text messaging service that offers 24/7 support for people struggling with mental health issues, on Friday.
"I am incredibly excited to be launching this service knowing it has the potential to reach thousands of vulnerable people every day," he added.
Princes William and Harry have focused in recent years on reducing the stigma around mental health issues. Part of that work has included speaking out about their own struggles after the death of their mother, Princess Diana, in 1997.
"I have probably been very close to a complete breakdown on numerous occasions when all sorts of grief and sort of lies and misconceptions and everything are coming to you from every angle,” Harry said in 2017, crediting William with encouraging him to seek out mental health support.
William and Harry joined with Kate in 2016 to launch Heads Together, an initiative to change the conversation on mental health.
Prince Harry, who served in the British Army, has focused on mental health for veterans, while William has focused on young men and fathers and Kate on young people, including bringing the Heads Together initiative to schools.
Their new Shout initiative is a partnership with the U.S.-based Crisis Text Line.
In the U.S., users can text Home to 741741 to receive immediate support. Users in the U.K. can text Shout to 85258.
William, Kate, Harry and Meghan began meeting with volunteers for Shout as far back as last November.
"Over the last few months, Shout has started working quietly behind the scenes," William said. "Harry, Meghan, Catherine and I have been able to see the service working up close and are very excited for its future."
He added, "We wanted to do our bit to make it easier for people to start to get the help they need. And, crucially, we wanted to back new innovative ways for people to have conversations wherever they are, at whatever time they need someone to talk to."
The public launch of Shout was announced just a few days after Meghan, the Duchess of Sussex, and Harry welcomed their first child, a son they named Archie Harrison Mountbatten-Windsor.
Harry and Meghan, who now live in Windsor, and William and Kate, who live in London at Kensington Palace, split their royal households earlier this year, but all four remain partners in their charitable initiative, The Royal Foundation.
Shout is a national service in the U.K. supported by The Royal Foundation. The service is powered by volunteers trained to "create a safe space for people experiencing mental health challenges," according to Kensington Palace.
The royals are hoping to grow the volunteer force from its current size of 1,000 to 4,000 by the end of the year.
"At the heart of this service will be an incredible national volunteer community, one which needs to grow to allow us to support more people in crisis," the two couples said in a statement. "We hope that many more of you will join us and be part of something very special.”
If you are in crisis, call the National Suicide Prevention Lifeline at 1-800-273-TALK (8255) or contact the Crisis Text Line by texting HOME to 741-741. You can reach Trans Lifeline at 877-565-8860 (U.S.) or 877-330-6366 (Canada) and The Trevor Project at 866-488-7386.