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In December 2020, the U.K. authorities reported to the World Health Organization (WHO) that a new COVID-19 variant, considered to be a variant under investigation from December 2020 (VUI-202012/01), was identified through viral genomic sequencing. Although several other mutants were previously reported, VUI-202012/01 proved to be about 70% more transmissible. Hence, the usefulness and effectiveness of the newly U.S. Food and Drug Administration (FDA)-approved COVID-19 vaccines against these new variants are doubtfully questioned. As a result of these unexpected mutants from COVID-19 and due to lack of time, much research interest is directed toward assessing secondary metabolites as potential candidates for developing lead pharmaceuticals. In this study, a marine-derived fungus <i>Aspergillus terreus</i> was investigated, affording two butenolide derivatives, butyrolactones I (<b>1</b>) and III (<b>2</b>), a meroterpenoid, terretonin (<b>3</b>), and 4-hydroxy-3-(3-methylbut-2-enyl)benzaldehyde (<b>4</b>). Chemical structures were unambiguously determined based on mass spectrometry and extensive 1D/2D NMR analyses experiments. Compounds (<b>1</b>–<b>4</b>) were assessed for their in vitro anti-inflammatory, antiallergic, and in silico COVID-19 main protease (M^pro) and elastase inhibitory activities. Among the tested compounds, only <b>1</b> revealed significant activities comparable to or even more potent than respective standard drugs, which makes butyrolactone I (<b>1</b>) a potential lead entity for developing a new remedy to treat and/or control the currently devastating and deadly effects of COVID-19 pandemic and elastase-related inflammatory complications.