Background: Cannabidiol (CBD) products are increasingly available to consumers in the United States and are subject to regulation by the U.S. Food and Drug Administration (FDA). CBD products cannot be marketed as unapproved new drugs with claims of therapeutic benefit. In addition, because CBD is the active ingredient in a FDA-approved CBD product, Epidiolex, CBD cannot be marketed as, or in, food products or dietary supplements. The FDA has issued Warning Letters to promote voluntary regulatory compliance. These letters provide insights as to the types of violations for CBD products detected in the U.S. market. Objective: The goal of this retrospective study was to content analyze Warning Letters issued by the FDA to identify illicit marketing of CBD products. Design: Warning Letters issued by the FDA between 2015 and 2019 were content analyzed using a deductive approach. We extracted year of issuance, issuing office, and claim types that are currently prohibited by the FDA, including (i) unapproved new drug, (ii) misbranded drug, (iii) false and/or misleading, (iv) FDA-approved/endorsed, (v) dietary supplement, and (vi) adulterated food product. In addition, we documented the disease or conditions the product claimed to affect, pharmacological effects, and location of violation. Results: Of the 39 Warning Letters issued, 97% were for violations made on company websites and 56% were for social media accounts. Almost all letters (97%) cited violations of marketing CBD as an unapproved new drug. These illicit therapeutic claims were made for >125 unique health problems, including cancer (87.2%), diabetes (71.8%), inflammation (66.7%), pain (66.7%), and arthritis (66.7%). The majority of letters (79.5%) also cited illicit marketing of CBD as a dietary supplement or food product. CBD was promoted as having 16 unique pharmacological effects, including anti-inflammatory (53.8%), anticancer (43.6%), and antipsychotic (30.8%). Conclusions: CBD products have been unlawfully advertised online as unauthorized drugs with health claims that promote therapeutic benefits and as dietary supplements. Efforts are needed to regulate and monitor illicit advertising so consumers are not misled about the risks and benefits of CBD use.