Aim: To estimate the disease specific prevalence of undiagnosed rheumatic and musculoskeletal diseases (RMDs) in Portugal and determine if people with undiagnosed RMDs have worse quality of life, physical function and higher health resources consumption, than people without RMDs. Methods: A subgroup analysis of EpiReumaPt was made that included all participants≥18 years evaluated by a rheumatologist. Participants were stratified into three groups: undiagnosed RMDs; previously diagnosed RMDs; non-RMDs. A descriptive analysis of the three groups was performed. To estimate the prevalence of undiagnosed RMDs, weighted proportion were computed considering the sample design. The three groups were compared (Undiagnosed RMDs vs non-RMDs; Previously diagnosed RMDs vs non-RMDs) for health related quality of life (HRQoL) (EQ5D), physical function (HAQ), mental health (HADS) and health resources consumption. The effect of being undiagnosed for these outcomes was assessed in multivariable models adjusted for age, gender, geographical region and years of education (reference: non-RMD). Results: A total of 3877 participants were included. The prevalence of undiagnosed RMDs was 29%. Compared to participants without RMDs, undiagnosed participants had lower HRQoL (EQ-5D: β (95% CI)=-0.07 (-0.103,-0.043)) and physical function (HAQ: β (95% CI)=0.10 (0.05, 0.15)), more anxiety (OR (95% CI)=2.3 (1.4, 3.7)) and depression symptoms (OR (95% CI)=1.4 (0.8, 2.4)). Undiagnosed RMDs participants were more likely to visit an orthopedist (OR (95% CI)=2.0 (1.1, 3.5)) and had a higher number of orthopedic appointments (IRR (95% CI)=2.5 (1.3, 4.9)) than participants without RMDs. Conclusion: Patients with undiagnosed RMDs are frequent in Portugal, have worse HRQoL, physical function and mental health than people without RMDs. Undiagnosed patients are nonetheless consumers of health resources and tend to seek help from specialties other than rheumatology. Increasing the awareness of RMDs might promote their early identification and treatment leading to both personal and societal benefits.