The Duke of Sussex said he wished his children Archie and Lilibet could have met his late mother, who “most certainly will never be forgotten”.
He was speaking in the US state of Colorado for Sentebale — his charity which is based in Africa and supports youngsters with HIV — before the 25th anniversary of her death in a car crash in Paris on August 31, 1997.
He said he hopes to do Diana proud every day.
Prince Harry has taken on the cause of supporting those with HIV/Aids through Sentebale, and by publicly having tests for the virus — including with pop star Rihanna in 2016.
Diana helped to change attitudes towards people with the disease, in part by being photographed shaking the hands of sufferers at a time many were concerned about how it spread.
“My dear friend Prince Seeiso [of Lesotho] and I founded Sentebale 16 years ago in honour of our mothers, with a mission to support many of the most vulnerable young people on the planet — and to give them care and help them thrive,” Prince Harry said.
“In Sesotho, the language of the Basotho — the word Sentebale means 'forget me not'.
“Next week is the 25th anniversary of my mother's death, and she most certainly will never be forgotten.
“I want it to be a day filled with memories of her incredible work and love for the way she did it.
“I want it to be a day to share the spirit of my mum with my family, with my children, who I wish could have met her.
“Every day, I hope to do her proud. She was tireless in her work to support and destigmatise those experiencing HIV/Aids. Fittingly, her favourite flowers were forget-me-nots.
“I hope we can remember my mother's legacy by recommitting to those we serve, whoever and wherever that may be.
“For Sentebale, our commitment is to ending HIV and providing meaningful care and support for the children and young people whose lives have been affected by this disease. Because they too will never be forgotten.”
Earlier on Thursday, Prince Harry rode to victory in an annual charity polo tournament in Aspen. Commentators praised his “superstar” performance in the Sentebale ISPS Handa Polo Cup.
Prince Harry and his older brother Prince William have long championed causes close to their mother's heart.