Background/Objectives: As the prevalence of depression and the use of antidepressants have risen steadily in the last decade, new treatment options are needed. Aloysia gratissima var. gratissima ethanol extract has previously shown antidepressant-like activity, and the present study was conducted to identify the active fraction and clarify the possible mechanisms of action. Methods: Tail suspension (TST) and forced swimming (FST) behavioral tests were performed, and possible mechanisms of action were elucidated using serotonergic, dopaminergic, adrenergic, and GABAergic system antagonists. UPLC-DAD-MS analyses were performed to identify compounds in active fractions, and molecular docking studies were carried out to determine the binding affinities of these compounds to serotonergic and dopaminergic receptors (5-HT_1A, 5-HT_2A, 5-HT₃, and D₂R). Results: Ethyl acetate and butanol fractions were found to decrease immobility time in FST. The reduction in immobility time during the FST caused by the ethyl acetate fraction was reversed by pretreating mice with WAY100635 (5-HT_1A antagonist), ketanserin (a 5-HT_2A antagonist, ondansetron (5-HT₃ antagonist), or haloperidol (D₂ antagonist). UPLC-DAD-MS analysis revealed a similar composition for the ethyl acetate and butanol fractions of A. gratissima var. gratissima. Pharmacokinetic predictions suggest that only a few of the identified compounds have the potential to permeate the blood–brain barrier, and molecular docking simulations showed that compounds such as 13-oxooctadecadienoic acid, ferulic acid, and coumaric acid have binding affinities to the druggable site of serotonergic and dopaminergic receptors. Conclusions: These results suggest that the Agg ethyl acetate fraction possesses antidepressant-like activities, altering dopaminergic and serotonergic system functions. Computational simulations also suggest that some of the identified compounds have binding affinities to the 5-HT_1A, 5-HT_2A, 5-HT₃, and D₂R receptors.