The author and former political adviser passed away at the age of 56 following complications from COVID after contracting the disease in March 2020.
Appearing on Monday's Good Morning Britain, Kate - who said her final goodbyes to Derek at his funeral on Friday - confirmed along with the royal couple, King Charles and a whole host of celebrities reached out with condolence letters.
"I had the most beautiful letter from David and Victoria Beckham – handwritten, very good handwriting… that was Darcey’s observation," the GMB host shared.
She then suggested that Kate had written to her while recovering in hospital, adding: "And from the royal family - Catherine and William sent a beautiful letter, and I know Catherine has been in hospital herself."
Kate continued: "Even the King - because there is somebody [who] knows about grief - that anticipatory grief where you know something has a risk of happening and how different it is when it does happen."
Despite revealing the heartwarming gesture, Kate confessed she had been "sort of sworn to secrecy" about the contents of the letters from the royal family.
"I just think it was lovely of them to reach out to me," she noted. "But it's lovely to know that those people understand those emotions. I think for all of us, it connects us all together."
Kate has previously expressed her gratitude to the royal family for their support throughout Derek's health battle.
At the Prince's Trust Awards at St James's Palace in October 2021, the Good Morning Britain presenter revealed that then Prince Charles wrote a letter to her, which included an offer of help from a royal physician for her husband.
Speaking to Charles at the time, Kate told him: "And you might not remember but you were very kind to me because my husband got very sick with Covid after the last Prince’s Trust awards. You wrote to me and gave me help, so thank you very much."
Derek was also present as Kate was made an MBE by Prince William at Windsor Castle in June 2023. The mother-of-two was recognised in the 2022 New Year Honours for her services to broadcasting, journalism and charity.
ITV star Kate made two documentaries detailing her family’s life as Derek battled the long-term effects of coronavirus, with both programmes winning National Television Awards in the authored documentary category.
She has also written two books – The Power Of Hope: The Moving Memoir From ITV’s Kate Garraway and The Strength Of Love: Embracing An Uncertain Future With Resilience And Optimism.
Kate announced her husband's death in a statement on her Instagram page, writing on 5 January: "I'm sad to have to tell you all that my darling husband Derek has passed away. As some of you may know he has been critically ill following a cardiac arrest in early December which, because of the damage inflicted by Covid in March 2020, led to further complications.
"Derek was surrounded by his family in his final days and I was by his side holding his hand throughout the last long hours and when he passed. I have so much more to say, and of course I will do so in due course, but for now I just want to thank all the medical teams who fought so hard to save him and to make his final moments as comfortable and dignified as possible.
"Sending so much love and thanks to all of you who have so generously given our family so much support. Rest gently and peacefully now Derek, my love, I was so lucky to have you in my life."