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In this study, the green macroalgae <i>Ulva rigida,</i> which contains 34.9% carbohydrates, underwent treatment with commercial hydrolytic enzymes. This treatment yielded a hydrolysate that contained 23 ± 0.6 g·L⁻¹ of glucose, which was subsequently fermented with <i>Saccharomyces cerevisiae</i>. The fermentation process resulted in an ethanol concentration of 9.55 ± 0.20 g·L⁻¹. The optimal conditions for ethanol production by <i>S. cerevisiae</i> were identified as follows: non-sterilized conditions, an absence of enrichment, and using an inoculum size of 118 mg·L⁻¹. Under these conditions, the fermentation of the green macroalgal hydrolysate achieved a remarkable conversion efficiency of 80.78%. The ethanol o/t ratio, namely the ratios of the experimental to theoretical ethanol produced, for <i>Scheffersomyces stipitis, Candida guilliermondii, Kluyveromyces marxianus,</i> and <i>S. cerevisiae</i> after 48 h of fermentation were 52.25, 63.20, 70.49, and 82.87%, respectively. Furthermore, <i>S. cerevisiae</i> exhibited the best outcomes in terms of ethanol production (9.35 g·L⁻¹) and conversion efficiency (80.78%) after 24 h (optimal time) of fermentation.