Temperature stress is a major environmental factor that limits the growth of cucumber (Cucumis sativus L.). Melatonin has been implicated in plant responses to abiotic stress; however, its transcriptional effects on cucumber under high- and low-temperature conditions remain poorly understood. In this study, cucumber seedlings were exposed to 25 °C, 40 °C, or 4 °C, with or without exogenous melatonin treatment at 75 μmol·L⁻¹, and their transcriptional responses were investigated using RNA sequencing (RNA-seq). Both high- and low-temperature treatments caused varying degrees of seedling wilting, whereas this phenotype was partially alleviated by melatonin application. Transcriptome analysis identified 7488 and 3400 differentially expressed genes in response to high and low temperatures, respectively, indicating both shared and temperature-specific transcriptional responses. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that pathways related to plant hormone signal transduction, phenylpropanoid biosynthesis, and protein processing in the endoplasmic reticulum were significantly enriched in multiple comparisons. Further analysis revealed that melatonin treatment altered the expression patterns of several hormone signaling-related genes and transcription factors, including members of the ERF, MYB, WRKY, and bHLH families. Notably, hormone signaling components exhibited distinct transcriptional responses under high- and low-temperature conditions. The expression trends of selected genes determined by quantitative real-time polymerase chain reaction (qRT-PCR) were generally consistent with the RNA-seq results. Collectively, exogenous melatonin application was associated with reduced wilting symptoms and coordinated transcriptional changes in hormone signaling pathways under temperature stress. These findings provide a basis for further elucidating the molecular mechanisms underlying melatonin-mediated temperature adaptation in cucumber seedlings.