The aim of this pilot study was to investigate the feasibility and effectiveness of a new psychoeducative intervention program (PEGASUS) for adults with ADHD and their significant others in a psychiatric outpatient context. At three outpatient psychiatric clinics, adults with ADHD and their significant others took part in PEGASUS, a psychoeducational program based on theories from cognitive behavioral therapy, neuropsychology, and cross-disciplinary evidence regarding ADHD. In total, 108 adults were allocated to treatment (51 with ADHD and their 57 significant others). Feasibility was evaluated regarding suitability of the intervention at a psychiatric outpatient clinic and treatment completion. Preliminary efficacy was evaluated per protocol from baseline to post-intervention (n = 41 adults with ADHD and 40 significant others). In a feasibility analysis, the intervention was judged to be a suitable treatment option for 94.5 % of all individuals with a primary diagnosis of ADHD at an outpatient psychiatric clinic. In total, 43 out of 51 allocated individuals with ADHD (84.3 %) completed the intervention. The corresponding figures for their significant others were 42 out of 57 (73.7 %). Knowledge about ADHD increased, and both the quality of relationships and psychological well-being improved from baseline to post-intervention in all participants. The significant others reported a reduction in the subjective burden of care, such as worry and guilt. The objective burden of care (such as financial problems) did not change. The findings support the potential value of psychoeducation for adults with ADHD and their significant others. An ongoing randomized controlled trial will generate further evidence concerning the PEGASUS program.