This systematic review aimed to identify the effectiveness of diabetes self-management education and support (DSMES) among adults within 12 months of diagnosis of type 2 diabetes mellitus (T2DM). We searched the Cochrane Library, MEDLINE, CINAHL, PsycINFO, ERIC, and other sources up to March 2019 to detect randomized controlled trials in the last decade based on the global guidelines' definition of DSMES. Data were categorized into biomedical, psychosocial, and behavioral outcomes and synthesized using a random-effects model. Quality of evidence was assessed using the Grading of Recommendations Assessment, Development and Evaluation system. We scrutinized 12 studies that enrolled 2,386 adults with newly diagnosed T2DM. Biomedical outcomes presented the pooled effects of HbA1c -0.21% (95% confidence interval, -0.38, -0.04), body weight -2.36 kg (-5.77, 1.05), and waist circumference -1.8 cm (-5.63, 2.04) when the data from the two studies with low risk of bias were combined (N = 1,082). Psychosocial and behavioral effects were inconclusive owing to mixed results from various scales and reporting. The quality of the body of evidence was low. DSMES within 12 months of T2DM diagnosis might affect patient-centered outcomes. Further studies with higher precision using standardized measurement methods are required.