The present study describes the phenolic profile of the leaves of 12 different willow species by UPLC®-RP18-PDA in terms of variability between species, years, harvest month, and among sexes. The analyzed compound classes include salicylic alcohol derivatives (SADs), caffeic acid derivatives (CADs), coumaryl alcohol glucosides (CAGs), flavan-3-ols, flavanones, flavanonols, flavones, and flavonols. Furthermore, the Salix species were chemotaxonomically classified on this basis, and correlations between the constituent classes were analyzed. The investigations indicate that the phenolic spectrum is highly species-specific but reveals no sex-specific variation. The flavan-3-ol content varied substantially among species, ranging from 2.1–36 mg/g DW and S. bicolor was determined to be a species of particular phytochemical interest, as it contained high concentrations of flavanonols (13 mg/g DM) and flavones (24 mg/g DW). Furthermore, at the end of the growth period, the secondary metabolite content was significantly higher and the compound classes CADs and flavan-3-ols were found to be significantly influenced by the harvest year. Chemotaxonomical classification revealed the separation of S. bicolor, S. purpurea, and S. caprea from the other species when compared with the generally accepted classification of the genus. This study contributes to a more comprehensive phytochemical characterization of willow species, which may support the development of willow extracts.