The London-based agency has held the account since 2007.
The brief is worth $2.1m a year over the next five years. It involves a multidisciplinary approach to advertising, experiential marketing, content and social media management, traditional and influencer PR and 'high-level trade co-ops'.
The agency has been asked to grow and develop California’s road-trip product and culinary appeal, concentrating on the family and young-adult markets.
UK visitor numbers to California are predicted to reach 812,000 by 2022, making the country an important tourism target market.
The account will be overseen by UK account director Emma Westman, formally the communications director, and travel trade director Mark Meredith. Strategy will be overseen by Jonah Whitaker, who has worked on the account for 11 years.
"To secure the Visit California account for the next five years, having held the account for the last 12, is kudos indeed to the expert knowledge and creativity we deliver out of Black Diamond," managing director Guy Chambers said.
"2020 looks set to be our busiest year of programming yet, and through our integrated marketing strategy we aim to attract new and returning visitors across the Golden State."
Visit California associate VP of global marketing Leona Read said the UK is the Golden State's second-biggest overseas market.
She added: "Through innovative programming with Black Diamond we've established a strong brand presence and retained market share. We look forward to the next five years."
Black Diamond has several travel trade and PR clients, including Japan National Tourism Organization, Travel Portland, Colorado Tourism Office, Visit Seattle, Visit Pittsburgh, Tourism Whistler, the Lufthansa Group and more than 18 regional California tourism boards.
Recently the agency won a brief to promote Denver tourism.