The Duke of Sussex mentioned his brother Prince William in his first official appearance since arriving in the UK last week.
In the video, he spoke movingly about his mother’s belief in the power of young people to “change the world” ahead of the unveiling of Diana’s statue.
He said: “Later this week, my brother and I are recognising what would have been our mum’s 60th birthday, and she would be so proud of you all for living authentic life with purpose and with compassion for others.
“Our mum believed that young people have the power to change the world.
“She believed in your strength because she saw it day in and day out and in the faces of young people exactly like you, she witnessed a boundless enthusiasm and passion.
“And I too see those same values shine through as it has done for 21 years now, The Diana Award carries her legacy forward by putting young people at the centre of our future – and this has never been more important.”
Harry added that the Princess of Wales would have been “proud” of more than 300 recipients of the Diana Award during a virtual ceremony.
The duke also paid tribute to the young leaders for “inspiring us with your brilliance, your determination and your compassion” and said he was preparing to mark what would have been his mother’s 60th birthday on Thursday.
In a pre-recorded message, the duke told the online audience: “Never be afraid to do what’s right. Stand up for what you believe in and trust that when you live by truth and in service to others, people will see that just as they did with my mum.”
Harry is quarantining at his Frogmore Cottage home ahead of the unveiling of the statue commissioned by Diana’s sons to honour their mother’s legacy.
A ceremony being staged on Thursday and attended by a handful of people including Diana’s close family.
The Diana Award was set up in memory of Diana, Princess of Wales, and her faith in the next generation to change the world for the better.
The charity, which runs antibullying and mentoring programmes, is unique as it also rewards young people who carry out inspiring work without expectation of reward.
Each year thousands of young people are nominated by individuals and their communities to highlight these selfless acts.
Additional reporting by the Press Association.