Braving the chill with bare legs, she paired the look with a burgundy clutch and heels.
Erdem is one of the Duchess’ go-to labels, taking to the London-based designer on a number of occasions.
Kate was joined at the event by Prince William and Prince Harry to outline their plans for their Heads Together foundation.
“The challenge that so many people have is not knowing how to take that first step of reaching out to another person for help,” Kate said during a speech.
“Admitting that they are not coping. Fear, or reticence, or a sense of not wanting to burden another, means that people suffer in silence – allowing the problem to grow larger and larger unchecked.
“William, Harry and I have been very privileged to witness in the course of our work countless examples of simple conversations that have changed lives, which were the first step on a path to recovery,” she continued.
“Just last week at the Anna Freud Centre, I heard from one mother how talking to a support worker was – in her words – like medicine. Simply by having someone there to have a conversation with helped her immensely.”