Diagnosis in summer had been shown to be associated with better survival from some cancers, but such studies on malignant melanoma where sun exposure is a risk factor for disease are rare. We evaluated seasonality in melanoma diagnosis and its effect on survival in Victoria, Australia using 26,060 cases reported to the population-based Victorian Cancer Registry during 1986-2004. To estimate the amplitude of the seasonal variation, we calculated the ratio of the number of melanoma cases diagnosed in summer to that in winter. Linear regression was undertaken to assess the variation in thickness, the main prognostic indicator for melanoma, by season of diagnosis adjusting for sex, anatomical site, year of diagnosis and age at diagnosis. We modeled excess mortality using Poisson regression controlling for possible confounders in order to study the effect of season of diagnosis on survival. An overall 46% summer diagnostic excess was evident (summer-to-winter ratio 1.46; 95% CI 1.41, 1.52). Results of linear regression showed that melanoma diagnosed in winter were thicker than those diagnosed in any other season (percentage difference in thickness -2.01, -6.97 and -10.68 for spring, summer and autumn, respectively; p < 0.001). In the Poisson regression model of relative survival, cases diagnosed in spring, summer or autumn had slightly lower excess mortality than those diagnosed in winter before adjustment for other variables, but after adjustment the excess mortality ratios were close to unity. Our findings do not support the hypothesis that melanoma cases diagnosed in winter have worse prognosis than cases diagnosed in other seasons.