Understanding the economic value of marine sanctuaries such as the Florida Keys National Marine Sanctuary (FKNMS) is important to justify public and private investments and to provide information to support management activities and understand their role in the nation’s blue economy. Very few studies have employed economic contribution analysis in examining economic value, even though it is more useful in influencing the behaviors of decision makers. This study therefore employs such a methodology to determine the economic importance of tourism and visitor spending in the sanctuary to Monroe County, Florida’s economy. Visitors who came to the area for ocean recreation and tourism spent a total of USD 1.7 billion, which translates to a contribution of 19,688 total jobs, USD 752 million in total labor income, USD 1.2 billion in total value added, and USD 2 billion in total output to the region. With regard to the spending of snorkelers and divers only, total spending is about USD 1.07 billion, contributing about 12,441 total jobs, USD 466 million in total labor income, USD 767 million in total value added, and USD 1.2 billion in total output. Ocean recreation is therefore an important economic driver in the region and efforts should be directed at protecting the diverse and sensitive ecosystem of the sanctuary.