Mangroves have great ecological value because of their unique intertidal ecosystems and increased sensitivity to environmental changes. This paper examines the natural colonisation and diversity of true mangroves and their associated species along the Champa Canal (Negua Diversion Canal) in the Purba Medinipur area of West Bengal, India. The study area was categorised into three distinct zones, based on salinity gradients and tidal influences: Zone I is characterised by marine dominance; Zone II represents a transitional phase; and Zone III is influenced by freshwater. Fieldwork encompassing species identification, water and soil analysis, and seasonal tidal data collection documented 10 true mangrove species across 8 genera and 6 families, including 35 mangrove associates from 31 taxa and 21 families between December 2023 and November 2024. Zone I exhibited the highest levels of salinity and species diversity, while Zone III displayed reduced variability due to the predominance of freshwater and the absence of tidal activity. In addition to soil characteristics such as electrical conductivity, pH, and nutrient levels, the water parameters exhibited significant seasonal and geographical variation. These include salinity (ranging from 2.17 to 18.9 ppt), pH (between 6.3 and 8.9), dissolved oxygen (from 3.48 to 6.5 ppm), and biochemical oxygen demand (ranging from 1.73 to 2.94 ppm). Notably, eleven non-estuarine mangrove species and six invasive alien plant species were documented; numerous invasive species rapidly occupied disturbed regions. The distribution of mangroves in the study area was closely associated with variations in environmental conditions. This comprehensive ecological study offers crucial baseline data necessary for the future preservation, monitoring, and management of mangroves in transitional estuaries.