Autism spectrum disorder (ASD) is characterised by impairments in social relating. A dominant model of ASD suggests that these social relating impairments are a result of a reduced capacity for theory of mind or mentalising, which itself has been linked to a neural network that includes dorsomedial prefrontal cortex (dmPFC). As individuals with ASD show reduced activity within dmPFC, we investigated whether modulation of this region with repetitive transcranial magnetic stimulation (rTMS) would improve social relating and mentalising in adults diagnosed with ASD. In a double-blind, sham-controlled clinical trial, adults with ASD underwent ten 15-minute sessions of high-frequency (5 Hz) deep rTMS to dmPFC over a two week period. Assessments of social relating and mentalising were conducted before the first deep rTMS treatment, immediately after the last deep rTMS treatment, and one-month after the last deep rTMS treatment. Participants in the active deep rTMS condition (n = 15) showed a significant reduction in social relating symptoms from pre-treatment to one-month post-treatment, and also showed lower social relating symptoms compared to those undergoing sham stimulation (n = 13) at both post-treatment assessments. There was also a significant reduction in social anxiety for those in the active deep rTMS condition. Experimental measures of mentalising, however, revealed no group differences. This research provides initial support for the use of non-invasive brain stimulation to improve social aspects of ASD. We are currently examining whether an extended stimulation protocol can yield more pronounced clinical benefits, and whether we can detect changes in glucose metabolism within social cognitive networks following deep rTMS.