This study sought to: (1) determine the prevalence of gambling disorder using the Diagnostic and Statistical Manual of Mental Disorders Version 5 (DSM-5; American Psychiatric Association in Diagnostic and statistical manual of mental disorders, American Psychiatric Publishing, Arlington, 2013) criteria; (2) identify the frequency and amount of money spent on gambling behaviors; and (3) determine demographic and treatment related predictors associated with gambling disorder in a substance using population. People receiving methadone maintenance treatment (N = 185) in an urban medical center consented to participate in the study. We used DSM-5 criteria to assess the 12-month prevalence of gambling disorder. Questions adapted from a previously developed measure were used to identify, describe and quantify the frequency of use and amount of money spent on gambling behaviors. Most participants were African-American (71.4 %), male (54.1 %), unmarried (76.8 %), unemployed (88.1 %) and had an income of <$20,000 (88.5 %). On average, participants were receiving 81.0 mg of methadone (SD: 22.8) daily. Nearly half (46.2 %) of participants met DSM-5 criteria for gambling disorder. Compared to those without gambling disorder, those with gambling disorder did not differ significantly with respect to demographic characteristics nor methadone dose. However, those with gambling disorder had been in methadone maintenance treatment for significantly less time. Those with gambling disorder were significantly more likely to report engaging in a variety of gambling behaviors. Given that the 12-month prevalence of DSM-5 defined gambling disorder was nearly 50 % future efforts to screen and treat gambling disorder in the context of methadone maintenance treatment are clearly warranted.