Background The main aim of post-mastectomy breast reconstruction is to improve the patient’s quality of life, which makes high-quality and validated patient-reported outcome measurements essential. None of the established instruments include evaluation of donor-site morbidity, such as impact on upper extremity and back function, when a latissimus dorsi (LD) muscle is used; and BREAST-Q LD questionnaire was therefore recently developed for this purpose. The aim of this study was to translate into Swedish and culturally adapt the BREAST-Q LD questionnaire’s two subscales, appearance and function, and perform a psychometric evaluation of the subscales in a Swedish population of patients. Methods This was a cross-sectional study. The questionnaire was translated according to established guidelines. The questionnaires were sent to all patients operated using an LD flap between 2007 and 2017. Internal consistency was assessed using Cronbach’s α. Inter-item correlations and corrected item-total correlations were calculated using the Pearson’s correlation coefficient. Convergent validity was evaluated by comparing the BREAST-Q LD questionnaire to the Western Ontario Osteoarthritis of the Shoulder Index, using the Spearman correlation coefficient. Test–retest reliability was tested with intraclass correlation coefficients (ICCs), and the coefficient of variation and Bland–Altman plots were drawn. Floor and ceiling effects were calculated. Known-group validation was tested by comparing scores from the patients and from normal controls using the Mann–Whitney U-test and by calculating eta squared effect size. Results The questionnaires were sent to 176 eligible patients and 125 responded (71%). The patients had been operated a mean of 6.6 years ago, and most (92%) had previous radiation. Internal consistency was satisfactory for both subscales. The correlation coefficients between questions were r > 0.30 for all items of both scales. The corrected item-total correlation coefficient ranged from 0.62 to 0.90. As hypothesised, the function scale was correlated with the WOOS “Physical symptoms” subscale. Reliability was adequate according to the ICCs. The ceiling effect threshold for the appearance scale was reached and that for the back scale was almost reached. There were significant differences between patients and controls, in the hypothesised direction. Conclusions The results of this study support a good internal consistency, convergent validity, test–retest reliability and known-group validation for the Swedish BREAST-Q LD questionnaire. However, it may be difficult to discriminate between patients with very mild and those with no symptoms using the appearance scale. Trial registration: ClinicalTrials.Gov identifier NCT04526561.