The hydrothermal carbonization (HTC) process provides biomass-derived hydrochar, a material that exhibits great potential as adsorbents for environmental pollution remediation. For the preparation of these materials a different feedstock including corn cob, grape pomace, Miscanthus, Paulownia leaves, and spent mushroom substrate can be utilized. The adsorption performances of hydrochars are significantly influenced by structural characteristics of the biomass, as well as the applied hydrothermal conditions, primarily the carbonization temperature. This overview outlines the performance, modifications, and potential applications of biomass hydrochar. It discusses diverse modifications, physical (to increase surface area) and chemical (to add oxygen-containing functional groups) that can further enhance the adsorptive properties of tested materials. Summarizing previous findings, it can be concluded that heteroatom doping and acid-base treatments significantly facilitate and enhance the removal of metal ions and dyes. Accordingly, the study emphasizes the critical role of hydrothermal treatment as a key process for producing biomass-derived hydrochar that can be tailored for specific adsorption applications. In general, three main research directions are recommended: integrating physico-chemical modification techniques, gaining a deeper understanding of adsorption mechanisms, and expanding the applications of biomass hydrochar. These directions present promising opportunities for the continued development and application of biomass hydrochar as an effective and sustainable adsorbent for environmental purification while simultaneously solving the problem of waste generation.