Runt-related transcription factor 3 (RUNX3) plays a pivotal role in tumor microenvironment and immune infiltration. However, the prognostic and immunological roles of RUNX3 in pancancer remain unclear. In the current study, we explored the expression profiles, prognostic landscape, and immune infiltration of RUNX3 in pancancer through a variety of online platforms, including HPA, ONCOMINE, UALCAN, GEPIA, PrognoScan, TCGA, TIMER, R2, and Reactome databases. In general, RUNX3 was widely expressed in tonsil, gallbladder, skin, spleen, lymph node, and bone marrow, and RUNX3 was frequently higher expression in tumor tissues compared to normal tissues. In prognostic analysis, the RUNX3 expression level was significantly correlated with the clinical outcomes of bladder cancer, blood cancer, brain cancer, breast cancer, colorectal cancer, lung cancer, and ovarian cancer. In mutation analysis, a total 72 mutation sites were located within amino acids 1 to 415 of RUNX3, including 65 missense sites and seven truncating sites, whereas the mutation frequency of skin cutaneous melanoma and uterine corpus endometrial carcinoma (UCEC) is relatively high (> 3%). In immune infiltration analysis, the RUNX3 expression level was significantly related to recognized markers and the immune infiltration levels of various types of immune cells in colon adenocarcinoma (COAD) and brain lower grade glioma (LGG). After that, 453 RUNX3 co-expressed genes were recognized in COAD, lymphoid neoplasm diffuse large B-cell lymphoma, LGG, and ovarian serous cystadenocarcinoma (OV). Pathway enrichment analysis revealed that RUNX3 co-expressed genes were remarkably enriched in immune system and tumor progression pathways. RUNX3 expression is associated with clinical prognosis, immune infiltration, and identified RUNX3 related pathways in a variety of tumors, which may serve as targets of promising prognostic markers and novel therapeutic targets for various human cancers.