Toads of the genus Melanophryniscus possess unique color patterns on the belly, which allows for individual recognition. Photo-identification has proven to be an efficient non-invasive technique to identify individuals for this genus. However, such color patterns are absent in newly metamorphosed individuals. We studied the development of the ventral coloration pattern and evaluated its persistence in Melanophryniscus montevidensis to determine the minimum size and age at which use of the color pattern is a trustworthy (i.e., as stable as in adults) identification method for this species. From spawns raised in the field, we obtained eight metamorphs and maintained them in semi-natural conditions to photograph their bellies. We visually analyzed the images to establish the stabilization point of the color pattern. Using the software Wild-ID, we calculated the similarity score between the images from the stabilization point with sets of images before and after stabilization. Similarity scores of adults from previous studies did not differ significantly from the scores of juveniles after the pattern stabilized, but they did differ significantly from the scores of juveniles compared to themselves at least 70 d before stabilization. The color pattern developed progressively and stabilized at a median of 220 d after metamorphosis, with a maximum snout-vent length of 13.2 mm, which we considered the minimum size for photo-identification purposes. Although we observed ontogenetic and individual variation, the pattern remained unchanged since just before the first year of age. Taking into account the threshold size we determined, photo-identification is a suitable method for ecological studies of this species.