Associative memory is fundamental to human cognition and has been strongly linked to neural oscillations in the theta frequency band. Rather than being confined to a single brain region, these processes are thought to emerge from dynamic interactions among temporal, frontal, and parietal areas, as proposed by the Temporo–Frontal–Parietal Network Model. The role of the parietal cortex plays a central and dynamic role in associative memory by supporting integrative processes that enable successful retrieval. The present study investigated whether transcranial alternating current stimulation (tACS) delivered on the left parietal cortex at individualized theta frequency (ITF) could modulate associative memory performance. Thirty healthy participants were randomly assigned to either a stimulation or sham condition. During the encoding phase of the Face and Scene Task (FAST), EEG recordings were collected. Each participant’s ITF, derived from their theta activity during the encoding task, was calculated. ITF tACS was delivered over the P3 site to the stimulation group, whereas the sham group received sham stimulation. Following encoding, all participants completed a recognition task. EEG, behavioral performance, and theta activity were compared across groups. The tACS group did not differ in recognition performance from those in the sham group. No significant effects were observed on spontaneous EEG with eyes open or directly on ITF. Time–frequency analyses revealed right-hemispheric dominance in the stimulation group and left-hemispheric dominance in the sham group within 100–400 ms. Comparisons between encoding and recognition phases suggested that stimulation modulated theta dynamics, contributing to hemispheric asymmetries. The tACS at ITF can change associative memory performance, although its effects on theta activity vary across spatial and temporal dimensions. The findings suggest that ITF tACS administered during encoding is associated with improved recognition performance compared to sham, highlighting the potential of personalized stimulation approaches to support memory processing.