The aim was to investigate trait social anxiety and social evaluative stress in autistic children and adolescents and Specific Learning Disorders (SLD). This was done by evaluating behavioral, subjective, and autonomic responses to the Trier Social Stress Test (TSST). Study 1 included 280 children and adolescents: 60 autistic without intellectual disability (ID), 70 SLD, and 150 non-diagnosed (ND) peers. Study 2 involved 55 participants: 15 autistic without ID, 15 SLD, and 20 ND. The sample was predominantly male. In Study 1, behavioral (quality of public speech) and subjective (valence, arousal, perceived competence, and worries) aspects of social stress were examined. Study 2 expands upon the first study, as physiological responses to social stress were also measured. Trait social anxiety was investigated using both parents’ and children’s reports in the two studies. Parents of autistic participants and with SLD reported higher trait social anxiety in their children than the parents of ND. No differences emerged from the participants’ self-reports, though those autistic and with SLD were assigned lower scores in the public speech quality than ND. Autistic children and adolescents reported lower arousal and higher perceived competence than ND, while those with SLD reported a lower perception of competence. Autistic participants had a decreased cardiac reactivity across the TSST when compared to SLD and ND. Our findings show unique patterns of responses within each group, confirming that the combined investigation of trait anxiety and social stress responses could be a valuable insight in research and clinical practice.