What constrains working memory capacity? Classic theories place visual working memory close to perceptual systems, with fixed limits. Yet, emerging evidence shows that visual working memory capacity is increased for real-world objects compared to simple or abstract stimuli. The present study demonstrates that this memory advantage arises from semantic understanding of real-world objects – contrary to classic perceptual accounts of this cognitive system. Using counterfeit objects generated by generative adversarial networks that match real objects in terms of object form and visual similarity, we show that improvements in behavioral performance and increases in neural delay activity emerge solely for semantically meaningful, real objects. Correlation analyses indicate that subjective familiarity ratings predict memory for real objects, whereas stimulus colourfulness predicts memory for artificial objects, suggesting distinct mechanisms support memory for different stimulus types. Thus, conceptual knowledge exerts strong effects on visual working memory, significantly extending current theories that emphasize low-level perceptual features.