Naming an object involves quick retrieval of a target word from long-term memory. Research using the semantic interference paradigm has shown that objects take longer to name when they are preceded by primes in the same semantic category. This has been interpreted as reflecting either competition during lexical selection or as an interference effect at a later, postlexical level. Since the behavioral finding has been a core argument for the existence of competition during lexical selection in naming, understanding its processing level is important for models of language production. We used MEG to determine the spatiotemporal localization of the interference effect. We also compared its neural signature to the effect of semantic relatedness in reading, in which relatedness is expected to speed up behavioral responses and reduce activity in the left superior temporal cortex at around 200–300 ms. This is exactly what we found. However, in naming, we observed a more complex pattern for our semantically related targets. First, the angular gyrus showed a facilitory pattern at 300–400 ms, likely reflecting aspects of lexical access. This was followed by a broadly distributed and sustained interference pattern that lasted until articulatory stages. More transient interference effects were also observed at 395–485 ms in the left STG and at ∼100–200 ms before articulation in the parietal cortex. Thus, our findings suggest that the semantic interference effect originates from both early and late sources, which may explain its varying localizations in previous literature.