As humans and social robots increasingly coexist, a crucial question remains: Do established social heuristics (such as the bystander effect) govern human behavior toward robots? Through two pre-registered experiments (N = 401 and N = 616), we demonstrate that the presence of bystanders consistently inhibits helping behavior toward robots in human-robot interaction (HRI), mirroring classic findings in human-human interaction. In Experiment 1, we found that when bystanders were present, individuals were less likely to help a social robot in need than if they were alone. In Experiment 2, we used video stimuli with higher ecological validity and incorporated a human victim as a control group, generalizing this effect and revealing its robust stability. While the presence of bystanders generally inhibited helping intentions (regardless of whether the victim was human or robot), the strength of the bystander effect did not differ between human and robot victims. These findings provide the first empirical evidence that the bystander effect can be replicated in human-robot interaction (HRI). Our work contributes to the extension of the application boundaries of the bystander effect and the construction of a harmonious human-robot symbiotic society.