Abstract Aims 3D echocardiographic (3DE) assessment of the left atrium (LA) is a new modality of potential clinical value. Age- and sex-based normative values are needed to benchmark these parameters for clinical use. Methods and results Of 4466 participants in the 5th Copenhagen City Heart Study, a prospective longitudinal cohort study on the general population, 2082 participants underwent 3DE of the LA. Healthy participants were included to establish normative values for LA strain, volume, and function by 3DE. The effects of age and sex were also evaluated. After excluding participants with comorbidities, 979 healthy participants (median age 44 years, 39.6% males) remained. The median and limits of normality (2.5th and 97.5th percentiles) for functional and volumetric measures were as follows: LA reservoir strain (LASr) 30.8% (18.4–44.2%), LA conduit strain (LAScd) 19.1% (6.8–32.0%), LA contractile strain 11.7% (4.3–22.2%), total LA emptying fraction (LAEF) 61.4% (47.8–71.0%), passive LAEF 37.7% (17.4–53.9%), active LAEF 37.4% (22.2–52.5%), LA minimum volume index (LAVimin) 10.2 (5.9–18.5) mL/m2, and LA maximum volume index (LAVimax) 26.8 (16.5–40.1) mL/m2. All parameters changed significantly with increasing age (P value for all <0.001). Significant sex-specific differences were observed for all parameters except active LAEF and LAVimax. Sex significantly modified the association between age and LASr (P for interaction < 0.001), LAScd (P for interaction < 0.001), LAVimin (P for interaction = 0.037), and total LAEF (P for interaction = 0.034) such that these parameters deteriorated faster with age in females than males. Conclusion We present age- and sex-specific reference material including limits of normality for LA strain, volume, and function by 3DE.