The validity and reliability of the six-item Arm Lymphoedema Physical Discomfort Scale was investigated. A postal survey was conducted among Japanese breast cancer survivors, and the data from eligible participants (n = 148) were analysed. Results indicated that the scale had sufficient internal consistency (Cronbach' Alpha = 0.76). The factor structure of the scale was examined using confirmatory factor analysis, which indicated a one-factor model with relatively good model fit indices. Factorial invariance was examined between help-seeking and non-helpseeking groups by using a multi-group confirmatory factor analysis. The results demonstrated partial factorial invariance between the groups. Latent mean differences revealed that participants who had reported severer physical discomfort were more likely to seek medical help than those who had not. Finally, convergent and divergent validities were examined using the physical domain in the WHO QOL-BREF, which indicated moderate correlations. It is concluded that the new measure has good internal consistency, factor structure, partial factorial invariance and divergent validity.