Proving that tweed can be sleek, Kate wore a fitted long-sleeved dress with a peplum detail at the waist while meeting the Rehabilitation of Addicted Prisoners Trust (RAPt), an addiction charity working to help prisoners addicted to drugs and alcohol.
Way back in February tweed was seen on the Fashion Week Autumn/Winter runways at Prada, Chanel, Max Mara, Marc Jacobs and Roksanda, to name but a few.
If you want to steal Kate's style - and why not? Let's face it we're unlikely to have to worry about wearing it to the same event as her - you can get her silver tweed Eaton dress from The Fold for £365.
Following the Duchess' visit, Kensington Palace shared an Instagram photo with a statement from the Duchess herself, which read: "I was reminded today how addictions lie at the heart of so many social issues.
"I saw again today that a failure to intervene early in life to tackle mental health problems and other challenges can have profound consequences for people throughout their lives.
"I am grateful to the women I met for sharing their difficult personal stories with me."