Dehydration is an important challenge, especially in elderly care. This study investigates the effects of a verbal statement of personalized social proof, i.e., an utterance that evokes participants’ gender identity, on the persuasiveness of a robot when encouraging people to drink more. In the scenario used, the robot suggests participants to drink more water once over the course of a 10–15 min interaction. At the end of the experiment, while filling out a questionnaire, participants who heard the utterance containing personalized social proof drink significantly more water than those who did not hear a persuasive utterance. Also the evaluation of the robot is influenced by the persuasive message. Thus, appealing to participants’ social identity when evoking social proof is effective in verbal human-robot interaction.