Karenia selliformis is a globally recognized dinoflagellate associated with harmful algal blooms and massive fish kills along southern Chilean coasts. Its toxicity varies with environmental factors and genetic diversity. While K. selliformis is traditionally linked to neurotoxins like gymnodimines (GYMs), analysis of the strain CREAN-KS02 from Chile’s Aysén Region (43° S) revealed no presence of toxins associated with this genus, such as gymnodimines, brevetoxins, or brevenal. Given the high toxicity and impact on marine life, our study aimed to functionally characterize the neurotoxic metabolites in the exudate of K. selliformis cultures. Cytotoxicity was evaluated using a Neuro-2a cell-based assay (CBA), determining an IC50 of 2.41 ± 0.02 μg mL−1. The incubation of Neuro-2a cells with the bioactive lipophilic extract obtained from the exudate of K. selliformis and the ouabain/veratridine couple showed activation of voltage-gated ion channels. Electrophysiological recordings on cultured mouse hippocampal neurons showed that the extract increased cell excitability in a dose-dependent manner, modulating action potential firing and exhibiting an opposed effect to tetrodotoxin. These findings indicate the presence of excitatory neurotoxic compounds affecting mammalian cells. This study provides the first mechanistic evidence of K. selliformis toxicity and highlights potential risks associated with its proliferation in marine environments.