Background and aims Anxiety and fear are increasingly seen as related, but distinct concepts, with anxiety describing a reaction to unclear or future threats, and fear to immediate threats. Anxiety and fear both play influential roles in pain. Yet, the two concepts have not been clearly distinguished in pain research. Their reported intensity differs between the sexes, and sex differences in the way pain anxiety and fear of (re)injury relate to pain intensity have been found separately in previous studies. However, they seem to be of a curious nature: In one study, pain anxiety was associated with elevated pain intensity in men, while in another, fear of (re)injury was associated with elevated pain intensity in women. This indicates a moderator effect of sex. The present study is the first to unite previous findings, and to show a more integrative picture, by examining and discussing this moderator effect of sex in a joint study of both pain-related anxiety and fear in both sexes. Methods In 133 patients (mean age 43.6 years, 62% female) with chronic low back pain (mean duration 7.7 years), sex differences were examined with correlations and a multiple linear regression analysis with interaction terms. Differences between subgroups of low and high anxiety/fear were explored via t -tests, following previous studies. Results Sex was supported as a moderator in the association of pain intensity with pain anxiety (PASS-20), and fear of (re)injury (TSK). Higher pain intensity was linked to higher pain anxiety only in men, and to higher fear of (re)injury only in women. A basic regression model with fear, anxiety, sex and disability as predictors ( R 2 = .14, F (4,123) = 3.24, p = .042) was significantly improved by the addition of the interaction terms Fear×Sex and Anxiety × Sex ( R 2 = .18, F (2,121) = 4.90, p = .001), which were both shown as significant predictors for pain intensity. Further t -tests revealed a significant difference in pain intensity between high and low anxiety in men ( t (47) = −2.34, p = .023, d = −.43), but not in women. Likewise, a significant difference in pain intensity between high and low fear showed in women ( t (80) = −2.28, p = .025, d = −.42), but not in men. Conclusions The results support a moderator effect of sex and suggest differential mechanisms between the sexes in pain anxiety and fear in development and maintenance of back pain. The current study is the first to report and analyse this moderator effect. As potential underlying mechanisms, evolution and socialization are discussed, which may elucidate why fear might be more relevant for pain in women, and anxiety more relevant for pain in men. Implications The results indicate the need for a more cautious conceptual separation of fear and anxiety in research. Future studies on fear and anxiety in pain should be aware of the distinction, in order to avoid reporting only half of the picture. The next step would be to solidify the results in different samples, and to examine whether a distinction between anxiety and fear in the sexes could have any benefit in pain treatment.