Lyme borreliosis (LB) is caused by Borrelia burgdorferi sensu lato (Bb-sl) and is the most common vector-borne disease in Europe. The objectives of this study were to determine the Bb-sl seroprevalence among the general Finnish adult population and to identify risk factors associated with Bb-sl-seropositive status. Two thousand sera from a nationwide health survey from 2011 were tested by whole-cell sonicate IgG ELISA, C6 peptide ELISA, and recomBead IgG 2.0 and test results were linked to a general health questionnaire. A multivariable logistic regression model was used to identify risk factors. The median age of the study population was 56 years (range 29–97) and the Bb-sl weighted seroprevalence was 3.9% (95% confidence interval (CI) 3.03–5.08). The weighted seroprevalence was significantly higher among males than females (adjusted odds ratio 1.91, 95%CI 1.21–3.04). The seroprevalence was highest in Southern, Central, and Eastern regions. The first Bb-sl seroprevalence study in Finland showed a seroprevalence of 3.9% (regional range 0.87%–6.12%). The results of this study can be used, together with previous data on LB incidence and spatial tick distribution, to target public health communication about preventive measures.