Plasmodium malariae is a neglected malaria parasite. It has wide geographic distribution and, although often associated with mild malaria, is linked to a high burden of anemia and nephrotic syndromes. Here, we report a cohort study conducted in the Kanchanaburi Province of Thailand during May 2013-June 2014 in which P. malariae infection was detected. Of the 812 study participants, two were found to be infected with P. malariae. One had an infection that led to acute malaria, but the other was positive for P. malariae at multiple visits during the study and apparently had chronic asymptomatic infection. Such persistent infection may explain how P. malariae has been able to thrive at very low prevalence and represents a challenge for malaria elimination.