Background/Objectives: MIDN (midnolin) is newly discovered method for critically regulating a ubiquitin-independent proteasomal degradation pathway. This study aims to examine the expression, prognostic value, genomic changes, interacting proteins, methylation status, and correlations with the tumor immune microenvironment of MIDN in various cancers. Methods: The GTEx, Depmap, GEPIA2, and Kaplan-Meier Plotter databases are applied to evaluate the MIDN level in tumor and normal tissues and the MIDN prognostic value in cancers. The genetic alterations of MIDN in cancers are investigated using the cBioPortal database. The STRING, GeneMANIA, DAVID, and Human Protein Atlas are harnessed to identify and analyze MIDN-interacted proteins. The Sangerbox 3.0 platform (a pan-cancer analysis module) is used to measure the correlations between the MIDN level and the tumor immune microenvironment, stemness, immune cell infiltration, tumor mutational burden, immune checkpoint genes, and RNA modification genes. Immunofluorescence, qRT-PCR, and Western blotting assays were used to evaluate the biological roles of MIDN in breast and gastric cancer cells. Results: MIDN expression was dysregulated in many cancers and associated with prognosis in several cancers, such as esophageal cancer. MIDN was mutated in 1.7% of cancers, and deep deletion was the dominant mutation type. NR4A1, PSMC1, and EGR1 were selected as MIDN-interacted proteins, and these four molecules were co-expressed in pancreatic cancer, liver cancer, urothelial cancer, melanoma, and breast cancer. MIDN expression was significantly correlated with the infiltration of CD8+ T cell, CD4+ T cell, B cell, macrophage, neutrophil, and DC both in prostate adenocarcinoma and liver hepatocellular carcinoma. The MIDN level was correlated with several immune checkpoint genes, such as VEGFA, and RNA modification genes such as YTHDF1, YTHDF2, YTHDF3, and YTHDC1 in cancers. Furthermore, in breast cancer cells, the downregulation of MIDN suppressed the colony formation abilities and lessened cell-cycle-associated and stemness-associated genes; in gastric cancer, the knockdown of MIDN diminished the mRNA levels of Nanog and LDHA. Strikingly, silence of MIDN upregulated FTO protein expression in both breast and gastric cancer cells. Conclusions: Our findings demonstrate the expression, prognostic value, mutation status, interacting proteins, methylation status, and correlations with the tumor immune microenvironment of MIDN. MIDN will be developed as a potential therapeutic target and a prognosis biomarker.