Abstract Purpose: Lower-grade gliomas (WHO grade II/III) have been classified into clinically relevant molecular subtypes based on IDH and 1p/19q mutation status. The purpose was to investigate whether T2/FLAIR MRI features could distinguish between lower-grade glioma molecular subtypes. Experimental Design: MRI scans from the TCGA/TCIA lower grade glioma database (n = 125) were evaluated by two independent neuroradiologists to assess (i) presence/absence of homogenous signal on T2WI; (ii) presence/absence of “T2–FLAIR mismatch” sign; (iii) sharp or indistinct lesion margins; and (iv) presence/absence of peritumoral edema. Metrics with moderate–substantial agreement underwent consensus review and were correlated with glioma molecular subtypes. Somatic mutation, DNA copy number, DNA methylation, gene expression, and protein array data from the TCGA lower-grade glioma database were analyzed for molecular–radiographic associations. A separate institutional cohort (n = 82) was analyzed to validate the T2–FLAIR mismatch sign. Results: Among TCGA/TCIA cases, interreader agreement was calculated for lesion homogeneity [κ = 0.234 (0.111–0.358)], T2–FLAIR mismatch sign [κ = 0.728 (0.538–0.918)], lesion margins [κ = 0.292 (0.135–0.449)], and peritumoral edema [κ = 0.173 (0.096–0.250)]. All 15 cases that were positive for the T2–FLAIR mismatch sign were IDH-mutant, 1p/19q non-codeleted tumors (P < 0.0001; PPV = 100%, NPV = 54%). Analysis of the validation cohort demonstrated substantial interreader agreement for the T2–FLAIR mismatch sign [κ = 0.747 (0.536–0.958)]; all 10 cases positive for the T2–FLAIR mismatch sign were IDH-mutant, 1p/19q non-codeleted tumors (P < 0.00001; PPV = 100%, NPV = 76%). Conclusions: Among lower-grade gliomas, T2–FLAIR mismatch sign represents a highly specific imaging biomarker for the IDH-mutant, 1p/19q non-codeleted molecular subtype. Clin Cancer Res; 23(20); 6078–85. ©2017 AACR.