Objective: To determine if treatment of co-occurring adult ADHD and Cannabis Use Disorder (CUD) with extended-release mixed amphetamine salts (MAS-ER) would be effective at improving ADHD symptoms and promoting abstinence. Method: A 12-week randomized, double-blind, two-arm pilot feasibility trial of adults with comorbid ADHD and CUD (n = 28) comparing MAS-ER (80 mg) to placebo. Main outcomes: ADHD: [greater than or equal to] 30% symptom reduction, measured by the Adult ADHD Investigator Symptom Rating Scale (AISRS). CUD: Abstinence during last 2 observed weeks of maintenance phase. Results: Overall, medication was well-tolerated. There was no significant difference in ADHD symptom reduction (MAS-ER: 83.3%; placebo: 71.4%; p = 0.65) or cannabis abstinence (MAS-ER: 15.4%; placebo: 0%; p = 0.27). MAS-ER group showed a significant decrease in weekly cannabis use days over time compared to placebo (p < 0.0001). Conclusions: MAS-ER was generally well-tolerated. The small sample size precluded a determination of MAS-ER's superiority reducing ADHD symptoms or promoting abstinence. Notably, MAS-ER significantly reduced weekly days of use over time.