Prevailing theories posit ensemble activities of neuronal populations as memory representation in the brain. Such ensemble activities may be detectable as spatiotemporal activation patterns emerging on the cortical surface. In this study, we analyzed the spatial patterns of electrocorticography (ECoG) in the monkey medial temporal lobe memory system. Three macaque monkeys were trained to memorize 5 pairs of associations between visual items, and were required to recall the associative memory in a pair association (PA) task. In each trial, one of the visual items was presented as a cue (1 sec). After a 1.5-sec delay period, up to 3 choice stimuli were sequentially presented. The monkeys were required to respond to the presentation of the paired-associate of the cue. The subjects underwent subdural implantation with a 128-channel ECoG electrode array covering a part of the perirhinal cortex. Multi-channel ECoG was recorded while the subjects performed the PA task. Task-related ECoG responses were evoked at a majority of the electrode channels. We analyzed spatial patterns of the ECoG responses during the cue period, and found that learned paired-associates evoked significantly similar pairwise activation patterns. Trial-by-trial pattern similarities were further demonstrated by a mutual decoding analysis based on multivariate machine learning. In that analysis, the activation patterns evoked in the trials with one particular visual item presented as a cue were classifiable by a machine-learning decoder trained on the activation patterns evoked in the trials where the paired-associate of that item was presented as a cue. Our results imply that highly overlapping cell assemblies represent associative memory via similar spatiotemporal cortical activation patterns.