Aims:  To inhibit the growth of the bloom‐forming cyanobacterium Microcystis aeruginosa using a rice straw extract. Methods and Results:  The cell numbers of the algal strain M. aeruginosa UTEX 2388 significantly decreased after treatment with different concentrations (0·01, 0·1, 1 and 10 mg l−1) of a rice straw extract for an 8‐day cultivation period. Among seven tested allelochemicals from rice straw, salicylic acid at 0·1 mg l−1 exhibited the highest allelopathic activity (26%) on day 8. A synergistic effect on algal growth inhibition was found when adding two or three phenolic compounds from the rice straw. Conclusions:  The growth of M. aeruginosa was inhibited by rice straw extract concentrations ranging from 0·01 to 10 mg l−1. This activity was due to the synergistic effects of various phenolic compounds in the rice straw. Significance and Impact of the Study:  The identification of rice straw as an effective material for the growth inhibition of M. aeruginosa implies it may have the potential to be used as an environment‐friendly biomaterial for controlling the algal bloom of M. aeruginosa in eutrophic water.