There is an ongoing debate on whether semantic interference effects in language production reflect competitive lexical selection or post-lexical response exclusion mechanisms driven by the response-relevant status of distractor words. To disentangle categorical relatedness and task-defined response relevance effects, we combined the picture-word interference task with the conditional naming paradigm in an orthogonal design. Participants were instructed to name objects typically located in or on the water (e.g. canoe) and refrain from naming objects typically located outside the water (e.g. bike), and vice versa. Semantic relatedness and response relevance of distractors were manipulated independently. Linear mixed model analyses were conducted with semantic similarity ratings of target-distractor as continuous predictor. The pattern of results revealed that semantic similarity beyond categorical relations is critical for interference effects to be observed, and not response relevance. These findings provide support for the assumption that lexical selection is competitive and that semantic interference effects in the PWI paradigm reflect this competition.