<p>Algae detection is crucial for environmental monitoring, water quality assessment and biofuel applications. Traditional methods, such as microscopy and spectrophotometry, often require complex sample preparation and expensive instrumentation. In the present study, we propose a one-dimensional photonic crystal (1D-PhC) based optical sensor for efficient and precise detection of algae based on refractive index variations. A symmetric 1D-PhC structure incorporating a defect layer is designed and analyzed for its ability to differentiate various algal species by leveraging their distinct optical properties, particularly their refractive index characteristics. The transfer matrix method (TMM) is used to model the photonic bandgap (PBG) and its sensitivity to changes in refractive index induced by different algae species. The results demonstrate that changes in the refractive index of water due to the presence of different algae lead to measurable shifts in the defect mode transmission peak. The proposed PhC sensor exhibits a distinct sharp defect mode shit within the visible region, making it suitable for real-time algae detection. The high quality factor (QF), narrow linewidth and sensitivity in the range of 115–125&#xa0;nm/RIU confirm its potential for practical sensing applications. This theoretical study provides a foundation for developing compact, cost-effective, and highly selective optical biosensors for environmental algae monitoring.</p>