Through preclinical tests, this study evaluated the effects of Agaricus brasiliensis consumption in rats with streptozotocin (STZ)-induced gestational diabetes mellitus (GDM) and considered its potential as a functional food. The perinatal period was evaluated considering the daily exposure to A. brasiliensis before and after GDM induction (Abb and Aba, respectively). Nutritional characterization of A. brasiliensis was performed (centesimal composition, β-glucans, phenolic compounds, and antioxidant activity). Concerning maternal reproductive development, the parameters assessed were maternal weight, oral glucose tolerance, hemogram, biochemical markers, redox status in blood, biochemical markers in amniotic fluid, and reproductive performance. Moreover, embryofetal development was evaluated. A. brasiliensis reduced hyperglycemia before STZ induction and maintained levels similar to the GDM group after STZ induction. A. brasiliensis also reduced alanine aminotransferase, aspartate aminotransferase, triglyceride, and cholesterol levels and increased high-density lipoprotein levels. The mushroom also presented antioxidant activity, improving parameters of oxidative stress. Furthermore, it protected the conceptus from actions promoted by STZ concerning external abnormalities. Thus, daily intake of A. brasiliensis in GDM suggests its potential as a functional food because the nutritional characterization of this mushroom indicated important antioxidant activity, improving lipid and glycemic functions and preventing oxidative damage from STZ.