In recent years, the online sale of kratom (Mitragyna speciosa), a Southeast Asian plant with both medicinal and psychoactive properties, has raised health concerns mainly due to the uncontrolled diffusion of adulterated kratom-related products. This exploratory study provides, for the first time, a snapshot of the availability of kratom products on the darknet which has been further validated by data searches on the surface web. A total of 231 listings of kratom across 23 darknet marketplaces were identified between March 2020 and October 2021. Among these, 40 were found actively sold across five markets by thirteen vendors. Listed items were mainly advertised as "safe" substitutes for medicinal products for the self-management of pain and other health conditions and offered in various forms (e.g., dry leaf powder, pills, capsules). Purchases were made using cryptocurrencies, with some vendors offering Pretty Good Privacy, and were shipped from Europe, Australia, the United States, and the United Kingdom. Goods sold by the same sellers also included illicit drugs and fraud-related products. Our study discovered a previously unknown diffusion of kratom products on the darknet mainly for self-treating a variety of medical conditions, suggesting the need for further research and immediate interventions to safeguard the well-being and health of kratom consumers.