Single-cell RNA sequencing (scRNA-seq) has transformed our understanding of tumours by enabling high-resolution profiling of their cellular composition. Traditionally perceived as masses of homogeneous cancer cells, tumours are now recognised as complex ecosystems shaped by the tumour microenvironment (TME), which includes diverse immune cells, cancer-associated fibroblasts and extracellular matrix components. scRNA-seq has revealed remarkable heterogeneity within the TME, identifying novel or rare immune cell subsets and delineating their dynamic functional states. In particular, it has illuminated intercellular signalling networks and temporal cell-state transitions that drive tumour progression and immune evasion. Moreover, the integration of scRNA-seq data with clinical information has highlighted its potential in improving patient stratification, biomarker discovery and therapeutic target identification. Here, we systematically summarise recent advances in applying scRNA-seq to dissect the TME, discuss the implications of these findings for immunotherapy resistance and precision oncology, and outline future opportunities for integrating scRNA-seq with emerging technologies to develop more effective and personalised cancer treatment strategies.