In the current society, boredom has a bad reputation. Among others, one reason is that boredom is a negative predictor for cognitive performance due to the detrimental effects on attention and engagement. Recently, however, the negative reputation has been challenged by studies showing that boredom seems to promote creativity. However, those studies examined the influence of incidental boredom on apparently unrelated creativity tasks, leaving it open to question what happens when the individual gets bored by the task itself. To examine this issue, participants performed six blocks of a creativity task, and we measured creativity performance and experienced boredom across blocks. Results showed that boredom increased in parallel with fluency performance. However, more detailed analyses showed that the fluency increase was not brought about by the increase in boredom but was fully accounted for by the effect of increased task practice. When controlling for practice effects, results revealed that boredom actually impaired fluency. Such a finding supports the view that boredom has a negative impact on cognitive performance and underlines the necessity for changes in educational settings to prevent boredom.