Diatom-diazotroph associations (DDAs) are ecologically significant symbioses that enhance new nitrogen inputs in oligotrophic marine ecosystems, yet their presence in Sri Lankan coastal waters has remained unconfirmed. This study is the first morphological record of the heterocystous, nitrogen-fixing cyanobacterium Richelia intracellularis as an endosymbiont within two diatom hosts, Rhizosolenia hebetata and Rhizosolenia formosa from the northern waters of Sri Lanka. Phytoplankton and water quality samples were collected monthly during 2023 from Mathagal, Kankesanthurai, and Akkarai. Richelia trichomes (8-14 cells per filament) with terminal heterocysts were observed exclusively at Mathagal (May) and Akkarai (August) within the periplasmic space of host diatoms. Water quality parameters at DDA-positive sites showed nitrate concentrations ranging from 0.05 to 1.43 mgL⁻¹ and phosphate from 0.04 to 0.16 mgL⁻¹. Morphometric characteristics of the Sri Lankan specimens (heterocyst diameter: 9.8–10.6 μm; trichome length: 40-71 μm) align closely with those reported from other Indian Ocean populations. This first record extends the known biogeographical distribution of Richelia-diatom symbioses into Sri Lankan waters and highlights their potential contribution to nitrogen cycling in nutrient-limited coastal environments. These findings underscore the need for integrated taxonomic and molecular investigations of DDAs in the northern Indian Ocean.