The Duke and Duchess of Cambridge at the mental health conference
Kate, 34, wore the peplum two-piece as she attended Guild of Health Writers event, which focuses on mental health, with her husband William.
The Anxiety Epidemic conference is supported by the Heads Together initiative – a campaign Prince Harry and the Duke and Duchess of Cambridge are behind.
The event, held at at Chandos House, aims to change the conversation the country is having on mental health into a positive one.
Paul Farmer, chief executive of Mind, is one of the guest speakers at the conference.
Duchess of Cambridge joins school for children's mental health week
Duchess of Cambridge attends charity event wearing sleek red suit
Earlier in the day Kate said she wants to teach her children the importance of respect, honesty and kindness.
The Duchess of Cambridge said making Prince George, the future King, and his sister Princess Charlotte aware of the value of these qualities was just as important as academic or sporting success.
The Duchess's comments came as she celebrated pupils with the "K factor", or kindness factor, during a visit to Mitchell Brook Primary in north-west London with her husband the Duke for the launch of Children's Mental Health Week.
Kate and William at the Guild of Health Writers conference
The annual event – which this year has the theme Spread A Little Kindness – is an initiative founded by the mental health charity Place2Be, which has Kate as royal patron, and runs counselling sessions at Mitchell Brook in Neasden.
Kate, who wore an outfit by Luisa Spagnoli, said: "My parents taught me about the importance of qualities like kindness, respect, and honesty, and I realise how central values like these have been to me throughout my life.
"That is why William and I want to teach our little children George and Charlotte just how important these things are as they grow up.
"In my view it is just as important as excelling at maths or sport."
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Kate helped launch Children's Mental Health Week on Monday
Speaking during a special school assembly where those with the K factor were recognised, the Duchess added: "People often ask me why I am so interested in the mental health of children and young people.
"The answer is quite simple – it is because I think that every child should have the best possible start in life.
"When I was growing up I was very lucky. My family was the most important thing to me.
"They provided me with somewhere safe to grow and learn, and I know I was fortunate not to have been confronted by serious adversity at a young age."