Perkinsus marinus is a significant pathogen in oyster aquaculture with expanding host and geographic ranges. This study evaluated the prevalence and infection intensity of P. marinus in major oyster farming regions across the USA, Mexico, Brazil, and Korea using a quantitative PCR (P. marinus–specific TaqMan quantitative PCR assay, Pm-qPCR) assay. Eastern oysters (Crassostrea virginica) were sampled from 7 USA sites, while Pacific oysters (Magallana gigas) were collected from Mexico, Brazil, and Korea. Compared to conventional PCR, the Pm-qPCR assay demonstrated significantly higher sensitivity, detecting P. marinus in >80.0% of samples at most sites and up to 100.0% in Port Norris, USA. Lower prevalence was found in Wellfleet, USA (58.0%) and Korean sites (63.0%–70.0%). The lowest infection intensities (<1,000 copies) were recorded at a high-energy open-water site in Buan, Korea. The assay’s specificity was confirmed using negative control oysters from Canada. These findings provide critical baseline data on P. marinus distribution and emphasize the superior diagnostic value of Pm-qPCR for early detection. As P. marinus spreads globally, sensitive and standardized tools like this assay are essential for disease surveillance and aquaculture biosecurity.