Recent studies revealed a pro-tumor effect of constitutive Type-1 interferons (IFN-I) production and the downstream signaling activity in several malignancies. In contrast, heterogeneity and clinical significance of the signaling activity in gliomas remain unknown. Thus, we aimed to depict the heterogeneity and clinical significance of constitutive Type-1 interferon (IFN-I) production and the downstream signaling activity in gliomas. We utilized multiplex immunofluorescence (mIF) on a 364 gliomas tissue microarray from our cohort. Moreover, we conducted bioinformatic analyses on the Cancer Genome Atlas (TCGA) and the Chinese Glioma Genome Atlas (CGGA) databases to investigate the heterogeneity and clinical significance of constitutive IFN-I signaling activity in gliomas. We observed high heterogeneity of the constitutive IFN-I signaling activity among glioma subtypes. Signaling increased with the WHO malignancy grade while decreasing in the gliomas with IDH mutations. Additionally, high IFN-I activity served as an independent predictor of unfavorable outcomes, and global DNA hypermethylation in IDH-mutant gliomas was associated with decreased IFN-I signaling activity. Positive correlations were observed between the IFN-I activity and glioma-associated inflammation, encompassing both anti-tumor and pro-tumor immune responses. Furthermore, the IFN-I activity varied significantly among tumor and immune cells in the glioma microenvironment (GME). Notably, a distinct pattern of IFN-I signaling activity distribution in GME cells was observed among glioma subtypes, and the pattern was independently associated with patient overall survival. Constitutive IFN-I signaling activity varies significantly among glioma subtypes and represents a potential indicator for increased glioma inflammation and unfavorable clinical outcomes.