Online grocery shopping has become increasingly important in daily life. Consumers use the helpfulness of electronic word‐of‐mouth (eWOM) to reduce uncertainty and risk in online grocery shopping. Emotions such as positive arousal (e.g., energised) and calmness (e.g., relaxed) are often found in the eWOM of food products. However, little is known about the influence of emotions on eWOM helpfulness and how the effects of emotions on the helpfulness of online reviews differ in cultures. This study conducted an experiment to examine the impact of positive arousal and calmness on the helpfulness of eWOM regarding healthy and unhealthy foods in Japan and the USA. We found that cultural factors influenced the effect of arousal on eWOM helpfulness. For Japanese participants, positive eWOM with low arousal was more helpful than that with high arousal. In contrast, for US participants, positive eWOM with high arousal was more helpful than that with low arousal. In both countries, credibility mediated the relationship between arousal and review helpfulness. This study extends the literature on eWOM helpfulness and affect valuation theory. Our findings provide insights for companies on using eWOM in advertising campaigns and for consumers on writing helpful eWOM.