The expansion of the Tanjung Jati Power Plant (stages 5-6), as part of Indonesia’s National Strategic Program, aims to meet the increasing demand for electrical energy. This study assesses the impact of the expansion on marine water quality, focusing on plankton and benthos as key bioindicators. Using a descriptive case study approach, we periodically measured various parameters—abundance, number of taxa, diversity index, uniformity index, and dominance index—from the start of construction to December 2022. Plankton data revealed a species range of 79-197 (with 19-23 species per sample), with abundance varying between 685,774 and 771,073 cells/m3. The diversity index ranged from 2.43 to 2.67, while the uniformity index varied from 0.64 to 0.71. The dominance index showed a gradual increase, ranging from 0.11 to 0.51. Benthos data showed 6-12 species (0-3 species per sample), with abundance ranging from 595 to 225 cells/m³. The benthos diversity index varied between 2.23 and 2.44, the uniformity index ranged from 0.9 to 0.94, and the dominance index ranged from 0.21 to 0.28. The results indicate moderate pollution in the surrounding marine waters due to the coal-fired power plant expansion. A decline in plankton and benthos abundance, taxa, and diversity, coupled with an increase in uniformity and dominance indices, suggests an ecological imbalance with potential long-term impacts on marine biodiversity.