Background: Although there are about 18,500 compounds have been isolated and reported from marine resources, the prominence of drug discovery research on marine microalgae is still very less while comparing to other natural resources. Hence, this investigation was designed, especially on some carotenoid-producing marine microalgae to evaluate their chemotherapeutic efficacies including antibacterial, antifungal, antioxidant, hemolytic, and anthelmintic activities. Objective: The objective of this research is to evaluate the suitability of the selected marine microalgae for biological activities and to perform the identification and quantification of fucoxanthin in their methanol extracts using high-performance liquid chromatography (HPLC)–diode-array detector technique. Materials and Methods: The methanolic extracts of all 10 marine microalgae were screened for antibacterial, antifungal, antioxidant, hemolytic, and anthelmintic activities. The fucoxanthin was identified and quantified by thin-layer chromatography and HPLC techniques, respectively. Results: Among the test microalgae, Isochrysis galbana (IG) showed the presence of the highest concentration of fucoxanthin (5.93 mg/g dry weight) and also exhibited notable antioxidant activities (86%) at 80 mg/mL. In antimicrobial activities, Dunaliella salina (DS) demonstrated the promising antimicrobial activities (minimum inhibitory concentration [MIC]: 40 mg/mL) against Gram-negative bacteria and fungi while Thalassiosira species showed activity (MIC: 40 mg/mL) against Staphylococcus aureus and fungi. It was also noted that all test extracts were resistant to Escherichia coli. In anthelmintic activity against Pheretima posthuma, there are two microalgae, namely IG and Chaetoceros gracilis, exhibited considerable anthelmintic potential (with P < 0.01). Conclusion: From this study, it was concluded that DS and IG could serve as a promising source for further investigation to discover new antimicrobial leads and also demonstrated the positive correlation with the carotenoid content.