This study explored the effects of comma insertion on the processing of garden path sentences in Japanese. In two experiments, participants read relative clause sentences containing two ambiguities: single versus relative clause and early‐opening (EO) versus late‐opening (LO) left clause boundaries. EO sentences were presented with or without a comma compatible with an EO boundary in Experiment 1 and, in Experiment 2, with an LO boundary. The results showed that the comma, whether compatible or incompatible with the correct clause boundary, decreased reading time for the relative clause's head noun, indicating that a comma helps readers avoid or recover from garden paths caused by relative clause structures. Conversely, a comma incompatible with a clause boundary increased processing costs of second ambiguity resolution (EO vs. LO). We concluded that punctuation affects processing of temporary ambiguity in Japanese as in languages with stricter punctuation rules; furthermore, readers depend strongly on punctuation for online processing of whole sentence structures. We also discuss the relationship between punctuation and (implicit) prosody.