The present study was designed to shed light on a topic rarely explored and to suggest possible ways to detect risk factors for the presence of suicidal ideation and behaviors in a sample of adult patients with Attention-Deficit Hyperactivity Disorder (ADHD). This study also explored the association between ADHD, affective temperaments, the presence of hypomania symptoms, and suicide risk. We hypothesized that (compared to healthy controls) (1) patients with adult ADHD would report more negative affective temperaments and more hypomania symptoms and (2) that they would have a higher suicide risk. The participants included 63 consecutive adult inpatients (18 women, 45 men) with ADHD and 69 healthy controls (42 women, 22 men). All participants were administered the Wender Utah Rating Scale (WURS), the Hypomania Check-List-32 (HCL-32), the Mood Disorder Questionnaire (MDQ), the Temperament Evaluation for Memphis, Pisa, Paris, and San Diego (TEMPS-A), and the Columbia-Suicide Severity Rating Scale (C-SSRS). Forty-six percent of the ADHD patients had an Axis 1 comorbid disorder. ADHD patients (compared to controls) more often reported suicidal ideation (46.0% vs. 5.9%, one-way Fisher exact test p < 0.001; phi = 0.46). ADHD patients and the controls also significantly differed in all the scales administered (with Cohen’s d between 0.92–4.70), except for the TEMPS-A Hyperthymia scale. A regression model indicated that ADHD was independently associated with higher scores of a negative temperaments/hypomania factor (Odd Ratio = 14.60) but not with suicidal ideation. A high incidence of suicidal ideation, comorbid psychiatric disorders, and negative affective temperaments was reported in adult ADHD patients, and clinicians should routinely assess risk factors for suicide among these patients.