The 30-year-old visited The Brink, an alcohol-free bar linked to Action on Addiction, where she received a card and bouquet of red roses from 8-year-old Jaqson Johnston-Lynch.
"Happy Valentine's Day, your Royal Highness," said Johnston-Lynch, whose mother works for Action to Addiction. "I'm sorry Prince William can't be here."
The Duchess of Cambridge became a patron of the organization in January. It is her second official solo outing since becoming a member of the royal family in April 2011.
She is expected to visit Alder Hey Children's Hospital Tuesday afternoon.