The diatom Isthmia enervis Ehrenberg 1838, while known from other regions, has never been formally recorded in Sri Lanka. This study provides the first confirmed record of I. enervis from the coastal waters of Northern Sri Lanka, discovered during a seasonal survey from March to May 2023. Phytoplankton samples were collected at three locations using a standard conical plankton net (mouth diameter: 41 cm, mesh size: 55 µm). The collected samples were preserved with 1% acidified Lugol's solution for subsequent microscopic analysis. After a thorough microscopic analysis, occurrence of I. enervis found exclusively at two locations at different times, namely Akkarai (March) and Mathagal (April-May), proved that I. enervis was both geographically and temporally restricted. This restriction is likely due to local environmental conditions, including temperature, salinity, and nutrients during the post-monsoon period. Light microscopy revealed the distinctive morphology of I. enervis, including very large, heteropolar frustules forming short zig-zag chains, and confirmed a remarkable polymorphism with co-occurring rhomboidal and trapezoidal cells. The population density showed a consistent increase, from 2,800 cells/L in March to a peak of 3,860 cells/L in May. Its presence was correlated with key environmental shifts, including rising water temperature suggesting a niche-specific adaptation to the post-monsoon transitional period. The discovery of I. enervis bridges a critical knowledge gap in the regional biogeography of marine diatoms. Its restricted and seasonal occurrence underscores the importance of targeted sampling in understudied regions to establish a baseline for monitoring this species, which may serve as a valuable bio-indicator in the face of environmental change in Northern Sri Lankan coastal ecosystems.