Various potentially toxic compounds associated with plastic (e.g., plastic additives) can enter the environment during plastic fragmentation and/or weathering. 1-Hydroxycyclohexyl phenyl ketone (1-HCHPK) is a widely used photoinitiator, e.g., in UV-radiation-curable technologies such as 3D-printing, plastic coatings and construction materials. 1-HCHPK may reach aquatic ecosystems via various waste-flows, including leaching from consumer goods. However, knowledge of its potential environmental hazard is scarce. In the present study, we addressed this data gap by assessing the acute and long-term toxicity of 1-HCHPK to freshwater microcrustaceans in environmentally relevant conditions using natural waters. The results showed that the acute toxicity of 1-HCHPK (L(E)C50) to pelagic Thamnocephalus platyurus and Daphnia magna and benthic Heterocypris incongruens ranged between 27 and 55 mg/L. Further, the long-term exposure of D. magna to low levels of 1-HCHPK (0.1 and 1.0 mg/L) did not affect ephippia hatching or organismal fitness, even in three successive daphnid generations. Thus, 1-HCHPK did not pose a hazard to the freshwater microcrustaceans at concentrations < 1 mg/L in the environmentally relevant conditions (i.e., multigenerational life cycle tests conducted in lake water at low chemical exposure concentrations). The tests employed in this study allowed for the environmentally relevant hazard assessment of emerging pollutants such as a plastic additive 1-HCHPK.