This study looked at the anticancer, antimicrobial, and antioxidant properties of a methanolic extract of Micractinium reisseri. In vitro antitumor activity of M. reisseri methanolic extract revealed an inhibitory effect against MCF-7 and HCT-116 (breast carcinoma and colon carcinoma) cell lines. Antimicrobial activity of M. reisseri methanolic extract was estimated against Gram-negative bacteria (Pseudomonas aeruginosa RCMB 010,049 and Escherichia coli RCMB 010,052), Gram-positive bacteria (Streptococcus pneumonia RCMB 010,010 and Bacillus subtilis RCMB 010,067) and fungi (Aspergillus fumigatus RCMB 02,568 and Candida albicans RCMB 05,036). The extract showed promising results against all the tested microbes except C. albicans which gave negative results. The algal extract showed 2,2-diphenyl-1-picrylhydrazyl (DPPH), ferric reducing and hydroxyl radical scavenging activity in a concentration-dependent manner with maximum scavenging activity at concentrations (0.001, 0.001, and 5 mg/ml) for each assay, respectively. High-performance liquid chromatography (HPLC) revealed that the active compounds are phenols, alkaloids, and flavonoids. In conclusion, M. reisseri methanolic extract exhibited effective anticancer, antimicrobial, and antioxidant activities. © 2023, The Author(s).