C5a receptor 1 (C5aR1) is associated with various inflammatory processes, the pathogenesis of immune diseases, and tumor growth. However, its role in the tumor microenvironment of gastric cancer (GC) remains unclear. In this study, the expression ofC5aR1in GC and normal gastric mucosa tissues was compared using data retrieved from the Gene Expression Omnibus (GEO) and The Cancer Genome Atlas (TCGA) databases, and the results were validated by in vitro qRT-PCR and immunohistochemical analyses. The relationship betweenC5aR1expression and the overall survival of patients with GC was analyzed using the Kaplan–Meier method. Subsequently, enrichment analysis was performed, and the signaling pathways were screened.C5aR1expression was also correlated with genes related to the immune checkpoint and immune cell infiltration. The results revealed thatC5aR1expression was enhanced in GC tissues compared to normal gastric tissues, and that patients with high expression ofC5aR1had a worse 10-year overall survival compared to those showing low expression ofC5aR1. Functional analysis revealed thatC5aR1is a gene related to the immune system and may play a crucial role in inflammatory and tumor immune responses. Additionally,C5aR1showed a positive correlation with most immune checkpoint-related genes and a negative correlation with natural killer cells, dendritic cells, and CD8+ T cells. Immune evasion risk was observed to be significantly greater in patients with higher expression ofC5aR1than in those with lower expression. The results of this study reveal thatC5aR1shapes a non-inflammatory tumor microenvironment in GC and mediates immune evasion.