Background: Although type 1 diabetes mellitus (T1DM) is recognized as a catastrophic disease among the different types of diabetes, it is often confusedly diagnosed in clinical practice and difficult in care. The objective of this study is to reach a multidisciplinary consensus for the establishment of clinical recommendations on T1DM to optimize its undoubtedly diagnostic evaluation and transitional care. Methods: Scientific evidence was reviewed by a committee of researchers, based on which recommendations related to T1DM diagnosis were formulated. A two-round method was conducted to compare the opinions of a panel of 32 specialists (adult endocrinologists [53.1%], pediatric endocrinologists [43.8%], a diabetes educator for child and adolescent [3.1%]) on these issues. Results: The panel reached consensus on two of the six items discussed. The four items on which no consensus was reached were related to autoantibody detection and age of onset. Up to 80% of the panelists favored items related to the glucagon test and diabetic ketoacidosis history for T1DM diagnosis. Consensus regarding transitional care through diabetes educators was established. Conclusion: The assessment conducted by experts on T1DM showed a high level of professional agreement regarding the proposed diagnostic and transitional care recommendations. A comprehensive analysis of the latest evidence is warranted for the items on which consensus was not established.