Over the last two decades, the identification of missense mutations in the alpha-synuclein (alpha-Syn) gene SNCA in families with inherited Parkinson disease (PD) has reinforced the central role of alpha-Syn in PD pathogenesis. Recently, a new missense mutation (H50Q) in alpha-Syn was described in patients with a familial form of PD and dementia. Here we investigated the effects of this novel mutation on the biophysical properties of alpha-Syn and the consequences for its cellular function. We found that the H50Q mutation affected neither the structure of free or membrane-bound alpha-Syn monomer, its interaction with metals, nor its capacity to be phosphorylated in vitro. However, compared with the wild-type (WT) protein, the H50Q mutation accelerated alpha-Syn fibrillization in vitro. In cell-based models, H50Q mutation did not affect alpha-Syn subcellular localization or its ability to be phosphorylated by PLK2 and GRK6. Interestingly, H50Q increased alpha-Syn secretion from SHSY5Y cells into culture medium and induced more mitochondrial fragmentation in hippocampal neurons. Although the transient overexpression of WT or H50Q did not induce toxicity, both species induced significant cell death when added to the culture medium of hippocampal neurons. Strikingly, H50Q exhibited more toxicity, suggesting that the H50Q-related enhancement of alpha-Syn aggregation and secretion may play a role in the extracellular toxicity of this mutant. Together, our results provide novel insight into the mechanism by which this newly described PD-associated mutation may contribute to the pathogenesis of PD and related disorders.