Caffeine is a natural psychoactive substance that belongs to a group of chemical compounds called purine alkaloids. Caffeine is found in various plants such as coffee, tea, cocoa, guarana, and yerba mate. It is often added to dietary supplements for its ability to increase metabolism and aid in weight loss. To determine the caffeine content in dietary supplements, a novel UHPLC method was developed, compatible with the rules of green analytical chemistry. The developed method used only water and ethanol for sample preparation and chromatographic separation on a short C18 column. The obtained method confirmed that caffeine may be analyzed using only environmentally friendly solvents, ethanol, and water. The developed method is characterized by its low limit of quantitation, equal to 0.047 µg/mL, and good reproducibility (a relative standard deviation lower than 1.1%). The obtained results show that the caffeine content in tested dietary supplements is 4–35% higher than the declared amount in most cases. In comparison, the caffeine content of the drug determined using this method was performed with an accuracy of 0.4% RSD.