Kidney renal clear cell carcinoma (KIRC) is the subtype pf kidney cancer having the highest mortality as well as the highest potential of invasion and metastasis. The expression of HADH, encoding a key enzyme in fatty acid β-oxidation, has rarely been reported to correlate with prognosis and immune infiltration in cancers. This study aimed to explore the prognostic value of HADH in patients with KIRC. Gene expression profiles and clinical data of KIRC patients were acquired from The Cancer Genome Atlas. We compared the expression of HADH between KIRC tissues and normal tissues. Then, the relationship between HADH expression and the clinicopathological characteristics (survival, age, gender, stage, and grade) of KIRC was explored. Data from several online databases and paraffin-embedded specimens from two cohorts were used for external validation (10 cases from Meizhou People's Hospital and another 75 cases from a tissue chip, with both cohorts including KIRC samples and paired normal tissues). We also predicted the fractions of tumor-infiltrating immune cells (TIICs) in various tissues using CIBERSORT. Next, we estimated the prognostic value of differences in TIIC proportions between the high and low HADH expression groups. Finally, gene set enrichment analysis (GSEA) was performed to explore the potential mechanisms by which HADH expression influences patient survival. The expression of HADH was significantly lower in KIRC tissue than in normal tissue. Decreased expression of HADH was significantly correlated with high histologic grade, advanced stage, and poor prognosis. The differential expression of HADH was validated at the protein level by immunohistochemistry. Multivariate Cox regression analysis indicated that HADH was an independent prognostic factor for KIRC. In addition, HADH expression was significantly associated with the accumulation of several TIICs, especially regulatory T cells. Finally, GSEA revealed that the transcriptome of the low HADH expression group was significantly enriched in genes involved in not only epithelial-mesenchymal transition and inflammatory response but also TNF-α, IL-6-JAK-STAT3, and interferon-γ signaling. In conclusion, our study demonstrated that decreased expression of HADH is related to poor prognosis and immune infiltration in KIRC; this finding may provide crucial information for the development of immunotherapies.