Octopuses and other cephalopods are uniquely valuable for comparative neuroscience. Octopuses, with the largest relative brains of any invertebrate, display sophisticated behaviors often ascribed only to complex vertebrates. While the capabilities of the octopus nervous system are similar to those of vertebrates, cephalopod brains evolved independently of the vertebrate brain. This combination of neural complexity and evolutionary distance from humans make cephalopods ideal for understanding how complex cognition can evolve. Despite the importance of this question, the genomic mechanisms underlying cephalopod cognition are not well understood. It has been hypothesized that nervous system complexity in the octopus may have evolved via several gene family expansions, including the sialin gene family. While sialins are involved in neurotransmission and are expanded in the octopus genome, their role in cephalopod evolution has not yet been explored. Here, we characterize the evolutionary history of the sialin family to better understand the octopus nervous system. We first identified the number of sialins in 39 bilaterian genomes, including 5 cephalopods. Next, we predicted the sialin count for each ancestor of these species using a gene birth-death model. Our reconstruction predicted a rapid expansion from 23 sialin copies in the ancestral cephalopod to 39 in octopods, suggesting that the sialin expansion occurred specifically in the octopus lineage. Future aims of this study include determining the rate of sialin evolution across cephalopods and predicting the function of sialins in the octopus nervous system. Our research will provide insights into the evolutionary roles of sialin in cephalopod neurobiology.