Here we present the ascidian Ciona intestinalis as an alternative invertebrate system to study Alzheimer’s disease (AD) pathogenesis. Through the use of AD animal models, researchers often attempt to reproduce various aspects of the disease, particularly the coordinated processing of the amyloid precursor protein (APP) by α-, β- and γ-secretases to generate amyloid beta (Aβ)-containing plaques. Recently, Drosophila and C. elegans AD models have been developed, exploiting the relative simplicity of these invertebrate systems, but they lack a functional Aβ sequence and a β-secretase ortholog, thus complicating efforts to examine APP processing in vivo. We propose that the ascidian is a more appropriate invertebrate AD model owing to their phylogenetic relationship with humans. This is supported by bioinformatic analyses, which indicate that the ascidian genome contains orthologs of all AD-relevant genes. We report that transgenic ascidian larvae can properly process human APP695 to generate Aβ peptides. Furthermore, Aβ can rapidly aggregate to form amyloid-like plaques, and plaque deposition is significantly increased in larvae expressing a human APP695 variant associated with familial Alzheimer’s disease. We also demonstrate that nervous system-specific Aβ expression alters normal larval behavior during attachment. Importantly, plaque formation and alterations in behavior are not only observed within 24 hours post-fertilization, but anti-amyloid drug treatment improves these AD-like pathologies. This ascidian model for AD provides a powerful and rapid system to study APP processing, Aβ plaque formation and behavioral alterations, and could aid in identifying factors that modulate amyloid deposition and the associated disruption of normal cellular function and behaviors.