is a commercially exploitable source of bioactive compounds and biofuels with potential applications in microalgae-based industries. Despite this, little taxonomical information is available. Therefore, proper identification and characterization are needed for the sustainable utilization of isolated microalgae. In this study, an axenically isolated unicellular green alga from the Geumgang Estuary, Korea was investigated for its morphological, molecular, and biochemical characteristics. The morphological characteristics were typical of G. emersonii. Molecular phylogenetic analysis of the 18S rDNA sequence verified that the isolate belonged to G. emersonii and was subsequently named G. emersonii GEGS21. It was isolated from brackish water, and its optimal growth temperature, salinity, and light intensity were at 28−32 °C, 0 M NaCl, and 130−160 µmol m−2 s−1, respectively. The strain thrived over a range of temperatures (5−40 °C) and withstood up to 0.5 M NaCl. The isolate was rich in omega-6 linoleic acid (C18:2 n-6, 26.3%) and palmitic acid (C16:0, 27.5%). The fuel quality properties were determined, and biodiesel from GEGS21 could be used as a biodiesel blend. Value-added carotenoids lutein (1.5 mg g−1 dry cell weight, DCW) and neoxanthin (1.2 mg g−1 DCW) were biosynthesized as accessory pigments by this microalga. The biomass of this microalga may serve as feedstock for biodiesel production as well as producing valuable ω-6 and carotenoids.