| Summary: | Three new diterpene alkaloids, (+)-8-epiagelasine T (<b>1</b>), (+)-10-epiagelasine B (<b>2</b>), and (+)-12-hydroxyagelasidine C (<b>3</b>), along with three known compounds, (+)-<i>ent</i>-agelasine F (<b>4</b>), (+)-agelasine B (<b>5</b>), and (+)-agelasidine C (<b>6</b>), were isolated from the sponge <i>Agelas citrina</i>, collected on the coasts of the Yucatán Peninsula (Mexico). Their chemical structures were elucidated by 1D and 2D NMR spectroscopy, HRESIMS techniques, and a comparison with literature data. Although the synthesis of (+)-<i>ent</i>-agelasine F (<b>4</b>) has been previously reported, this is the first time that it was isolated as a natural product. The evaluation of the antimicrobial activity against the Gram-positive pathogens <i>Staphylococcus aureus</i>, <i>Streptococcus pneumoniae</i>, <i>Enterococcus faecalis</i> showed that all of them were active, with (+)-10-epiagelasine B (<b>2</b>) being the most active compound with an MIC in the range of 1–8 µg/mL. On the other hand, the Gram-negative pathogenes <i>Acinetobacter baumannii</i>, <i>Pseudomonas aeruginosa</i>, and <i>Klebsiella pneumoniae</i> were also evaluated, and only (+)-agelasine B (<b>5</b>) showed a moderate antibacterial activity with a MIC value of 16 μg/mL.
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