Gambling disorder (GD) is a recurrent and persistent problematic gambling behavior that impairs multiple areas of an individual's life. GD can persist through two modes: online or offline. This study aims to compare sociodemographic, clinical, and psychological characteristics between treatment-seeking online and offline gamblers and analyze the effect of the gambling mode (online or offline) on anxiety, depression, impulsivity, and debts. Seventy-nine treatment-seeking gamblers (96.2% males), who were simultaneously receiving treatment at a specialized Pathological Gambling and Behavioral Addictions Unit, participated in this study. The sample was divided into two subsamples: online ( = 29, 100% males) and offline ( = 50, 94% males); the characteristics of these two groups were compared and analyzed using Chi-Square test (χ), -Test or Mann-Whitney -test ( < 0.05). Multiple linear regression analyses were performed to determine the effects of gambling mode on significant variables (lack of premeditation and debts). The online sample with a mean age of 29.4 years mainly chose to engage in sports betting (45%, < 0.05) and showed a higher lack of premeditation levels (25.8 points, < 0.05) than the offline sample. In addition, the online sample was younger with respect to their onset to gambling (20.2 years, < 0.05) and the beginning of their gambling problems (25 years, < 0.05) compared to the offline sample. Online gambling increased the levels of lack of premeditation by an average of 5.43 points compared to offline gambling ( < 0.05). Accumulated debts of the online sample were lower (€11,000) than those of the offline sample (€12,000). However, the interaction between age and gambling mode revealed that online gamblers increased their debt amounts with age at an average increase of €2,726.33 per year compared to offline gamblers ( < 0.05). No significant influence of gambling mode was found on GD severity, anxiety, and depression levels. Gambling mode has a significant relationship with lack of premeditation-a component of impulsivity-and accumulation of debts in treatment-seeking people with GD; however, no relationship was found with the rest of the variables analyzed. Future research with larger samples is needed to confirm these findings.