Background: The treatment gap for mental disorders remains a challenge worldwide. Identifying reasons for nontreatment may contribute to reducing this gap.Aims: To evaluate sociodemographic and clinical factors associated with use and barriers to treatment in Portugal.Method: Data from the 2009 National Mental Health Survey were used. Participants reported 12-month treatment and reasons for nontreatment. Logistic regression models analysed the association between sociodemographic (education; employment; income; marital status) and clinical variables (mental disorder diagnosis; disability) with treatment and type of barriers (low perceived need; structural; attitudinal).Results: The majority of participants with a mental disorder was not treated. Treatment was more common among participants with mood disorders (OR = 4.19; 95% CI: 2.72-6.46), and disability (OR = 2.43; 95% CI: 1.33-4.46), and less common among single participants (OR = 0.38; 95% CI: 0.20-0.70) and those with basic/secondary education (OR = 0.42; 95% CI: 0.24-0.73). Attitudinal barriers were more likely among participants with none/primary (OR = 2.90; 95% CI: 1.42-5.90) and basic/secondary education (OR = 1.70; 95% CI: 1.01-2.85), and less likely among those with substance use disorders (OR = 0.27; 95% CI: 0.10-0.70). Low perceived need was higher among single people (OR = 1.77; 95% CI: 1.01-3.08), and lower among those with anxiety (OR = 0.50; 95% CI: 0.28-0.90) and mood disorders (OR = 0.16; 95% CI: 0.09-0.30). Unemployed participants had higher odds of reporting structural barriers (OR = 3.76; 95% CI: 1.29-10.92).Conclusions: This study identifies factors associated with nontreatment, providing useful evidence to develop policies and effective interventions.