The decline of malaria in Southeast Asia has complexified how fever is perceived and what actions are taken towards it. Using a mixed-methods design, we investigated the concept of fever and the determinants influencing health-seeking behaviour among migrants on the Thai-Myanmar border, where rapid economic developments collide with precarious political and socio-economic conditions. Fever corresponded with a high diversity of terms, symptoms and believed causes. The qualitative analysis identified distance and legal status as key barriers for accessing health care. The quantitative analysis further investigated determinants influencing health-seeking behaviour: living near a town where a cost-free clinic operated was inversely associated with seeking care at health posts (adjusted odds ratio [aOR], 0.40, 95% confidence interval [95% CI] [0.19-0.86]), and public hospital attendance (aOR 0.31, 95% CI [0.14-0.67]). Living further away from the nearest town was associated with health posts attendance (aOR 1.05, 95% CI [1.00-1.10] per 1 km). Having legal status was inversely associated with cost-free clinics attendance (aOR 0.27, 95% CI [0.10-0.71]), and positively associated with private clinic (aOR 2.56, 95% CI [1.00-6.54]) and public hospital attendance (aOR 5.15, 95% CI [1.80-14.71]). Fever conception and believed causes are context-specific and should be investigated prior to any intervention.