We report a dengue virus (DENV) co-infection in a Belgian traveler after a three-weeks holiday to Thailand. The patient recovered well without any complication. The infection was diagnosed by NS1 antigen testing and the concurrent presence of serotype DENV1 and DENV2 was demonstrated by reverse transcriptase polymerase chain reaction (RT-PCR) in acute phase serum sampled three days after symptoms onset. The predominant DENV1 serotype was identified as genotype I, lineage Asia-3 by sequencing. To our knowledge, this is the first time that a dengue co-infection is reported in a European traveler. The co-infection accounts for 1.0% of the total number of RT-PCR-positive samples (n=105) diagnosed in the reference laboratory of Belgium between 2008 and 2013. We expect that the number of reports on acute co-infections will increase in the coming years considering the increasing number of regions that are progressively becoming hyperendemic, especially in Southeast Asia.