The Duchess wrote: "Today on Armed Forces Day, William and I would like to pay tribute to the brave men and women, past and present, serving in all of our armed forces, at sea, on land and in the air, here in the UK and around the world.
"Thank you for all you and your families sacrifice to keep us safe. Last year, I was honoured to spend time with the @BritishArmy to see how they train serving personnel and new recruits.
"It was wonderful to see first-hand the many important and varied roles the military play day in, day out to protect us all, and I look forward to discovering more about the @RoyalNavy and @RoyalAirForceUK in due course."
The photos show Kate at the Pirbright Training Academy in November where she spent time with the 101 Operational Sustainment Brigade at Abingdon Airfield having a look at how new recruits were trained.
She also took part in training exercises and spent time speaking to personnel about their experiences in the British Army.
Photos show the Duchess and mother-of-three helping repair machinery and beaming as she wore an army helmet and rode in an army vehicle.
She concluded the post by signing it off with "C" for Catherine.
Previously called Veterans' Day, Armed Forces Day has been running since 2006, and although it is not a public holiday, it is still supported and celebrated through the UK.
It was renamed in 2009 to include all of the military community – not just veterans- and has since been held on the last Saturday of June every year.
Earlier this week Kate and hubby Prince William visited the new Windrush Monument at Waterloo station and Kate shared a special moment with a young girl.
Kate reached out her hand to the girl in school uniform, and the girl tightly took her hand before the pair shared a special moment and exchanged words.
