Gastric cancer (GC) is a prevalent malignancy in China, posing significant health challenges. The persistently high tumor incidence and the rising number of tumor survivors may elevate the risk of multiple primary malignancies (MPMs) in GC patients.

We conducted a retrospective study involving 11,045 patients diagnosed with gastric adenocarcinoma at National Cancer Center of China between 2010 and 2017. A 6‐month interval distinguished synchronous MPMs (SMPMs) from metachronous MPMs (MMPMs).

Of all GC patients, 443 (4.01%) had MPM, with 75.85% being males and an average age of 60 at initial diagnosis. Among the MPM cases, 415 (93.68%) had two primary cancers (53.98% synchronous and 46.02% metachronous), 27 (6.09%) had three, and 1 (0.23%) had four, totaling 472 extra gastric cancer sites. Esophageal, colorectal, and lung cancers were the most prevalent concomitant malignancies, with esophageal cancer more frequent in synchronous cases and breast cancer (observed only in females) more frequent in metachronous cases (both p < 0.001). In patients with two primary cancers, the SMPM subset included more males (p < 0.001), with higher rates of smoking (p = 0.012), alcohol consumption (p < 0.001), and higher T stage at first diagnosis (p < 0.001). The MMPM subset included more females (p < 0.001), was younger at first diagnosis (p < 0.001), and more frequently underwent surgery after first diagnosis (p < 0.001).

MPM occurring in patients with GC predominantly involve the esophagus, colorectum, and lung. Elderly male patients with smoking or alcohol use represent a higher‐risk group and warrant vigilant surveillance. This study provides a clinical and epidemiologic basis for future research on MPM in GC.