In primary cultures of rat hippocampal neurons, an ethanol extract of the red alga Gracilariopsis chorda (GCE) and its active compound arachidonic acid (AA) significantly increased the densities of dendritic filopodia and spines, promoted the expression of presynaptic vesicle protein 2 (SV2) and postsynaptic density protein 95 (PSD-95), induced robust synaptogenesis, and increased the expression of cell division control protein 42 (CDC42) and actin-related protein 2 (ARP2), which are important for actin organization in dendritic protrusions, and facilitated presynaptic plasticity by increasing the size of the synaptic vesicle pool at presynaptic nerve terminals.