Abstract: Seasonal effects on photosynthetic pigments, nutrients, flavonoids, and polyphenol and antioxidant activities of Abrus precatorius L. were studied in three different growing seasons within a year. An experiment was conducted during January, 2019 to March, 2020 at the Botanical Garden, University of Chittagong. The highest foliar photosynthetic pigments, nutrients were observed in monsoon (June-August) and the lowest in late monsoon (September-November) whilst maximum flavonoid content was determined in the leaf extract of early monsoon (March-May) and minimum in late monsoon (September-November) respectively. In contrast the highest polyphenol content and antioxidant activity were estimated in the leaf extract of late monsoon (September-November) and minimum in early monsoon (March-May) respectively. The present study concludes that monsoon (June-August) is suitable for efficient photosynthesis as well as nutrient accumulation in Abrus and early monsoon (March-May) is favourable for flavonoid synthesis. Considering the amount of the all studied elements late monsoon (September-November) is the right time for harvesting the leaves of Abrus to be obtained maximum polyphenol and antioxidant activity for medicinal use.