Vocabulary is crucial for an array of life outcomes and is frequently impaired in developmental disorders. Notably, ‘poor comprehenders’ (children with reading comprehension deficits but intact word reading) often have vocabulary deficits, but underlying mechanisms remain unclear. Prior research suggests intact encoding but difficulties consolidating new word knowledge. We test the hypothesis that poor comprehenders’ sleep‐associated vocabulary consolidation is compromised by their impoverished lexical‐semantic knowledge. Memory for new words was tracked across wake and sleep to assess encoding and consolidation in 8‐to‐12‐year‐old good and poor comprehenders. Each child participated in two sets of sessions, one beginning in the morning (AM‐encoding) and the other in the evening (PM‐encoding). In each case, they were taught 12 words and were trained on a spatial memory task. Memory was assessed immediately, 12‐ and 24‐hr later via stem‐completion, picture‐naming, and definition tasks to probe different aspects of word knowledge. Long‐term retention was assessed 1–2 months later. Recall of word‐forms improved over sleep and postsleep wake, as measured in both stem‐completion and picture‐naming tasks. Counter to hypotheses, deficits for poor comprehenders were not observed in consolidation but instead were seen across measures and throughout testing, suggesting a deficit from encoding. Variability in vocabulary knowledge across the whole sample predicted sleep‐associated consolidation, but only when words were learned early in the day and not when sleep followed soon after learning. Poor comprehenders showed weaker memory for new words than good comprehenders, but sleep‐associated consolidation benefits were comparable between groups. Sleeping soon after learning had long‐lasting benefits for memory and may be especially beneficial for children with weaker vocabulary. These results provide new insights into the breadth of poor comprehenders’ vocabulary weaknesses, and ways in which learning might be better timed to remediate vocabulary difficulties.