Nanoplastics generated from plastic materials have been of utmost concern to human and ecosystem health. Because of their tiny size, nanoplastics can easily penetrate biological barriers and enter cells, resulting in unknown diseases. Although there is growing interest in detecting nanoplastics, and many techniques have been utilized to detect nanoplastics, a fast, easy, and reliable method is still required. Hence, an efficient method based on surface-enhanced Raman scattering (SERS) with periodic Ag nanoarray (AgNA) substrates shows promising potential in detecting polystyrene nanoplastics (PSNPs) with high sensitivity. By thermally evaporating Ag onto periodic anodic aluminum oxide (AAO) templates, the AgNA with high SERS performance is fabricated. The SERS performance of the AgNA is studied by changing its height. By directly drop-casting PSNPs onto the AgNA, the limit of detection (LOD) for PS becomes 10 μg/mL. Moreover, the AgNA is applied to detect PSNPs under different real water conditions with good recovery rates. Thus, this easily fabricated and highly sensitive SERS substrate provides a fast, efficient, and practical way to detect PSNPs in aqueous media.