The Mediterranean diet is associated with multiple health benefits. Yet, no tool has been specifically developed to assess adherence to the 'traditional' Mediterranean diet and cuisine within a Western cohort, and validated for online use. We tested the reliability and validity of online administration of the Mediterranean Diet and Culinary Index (MediCul) among middle-aged and older adults. Participants were recruited in January⁻March 2017 from the 45 and Up Study, completing MediCul twice. Test-retest reliability was assessed using the paired -test, intra-class correlation coefficient (ICC) and Bland-Altman plot. Validity was tested against a three-day food record (FR)-derived MediCul score using Bland-Altman and nutrient trends across the MediCul score tertiles. Participants ( = 84; 60% female; 65.4 years (SD = 5.9)), were overweight (BMI 26.1; SD = 4.0) with 1.7 (SD = 1.5) chronic illnesses/conditions. Sequential MediCul tool scores were 56.1/100.0 and 56.8/100.0, respectively ( = -1.019; = 0.311). Reliability via ICC (ICC = 0.86, 95% CI: 0.789, 0.910, < 0.0001) and Bland-Altman was good. In Bland-Altman validity analyses, the tool over-reported FR MediCul score by 5.6 points with no systematic bias ((y = 8.7 - 0.06*x) (95% CI: -0.278, 0.158, = 0.584)). Nutrient trends were identified for MediCul consistent with expected Mediterranean patterns. Online MediCul administration demonstrated good reliability and moderate validity for assessing adherence to a 'traditional' Mediterranean pattern among older Australians.