Background/Objectives: Glioblastoma (GBM) is the most common and aggressive primary brain tumor in adults, with a poor prognosis and limited therapeutic options. This study aimed to repurpose methylstat, a selective histone demethylase inhibitor, as a novel anti-glioma agent. We characterized its anti-proliferative efficacy, elucidated mechanisms of cell cycle regulation, and evaluated its blood–brain barrier (BBB) permeability potential. Methods: Compounds with transcriptional profiles enriched for cell cycle arrest and tumor-suppressive pathways were identified via Connectivity Map (CMAP) analysis. Methylstat was selected based on its high connectivity score and favorable physicochemical properties. In vitro assays were performed to evaluate its effects on cell viability, proliferation, cell cycle progression, and expression of related molecular markers in U251 and HOG glioma cell lines. Molecular docking and 200 ns molecular dynamics (MD) simulations were performed to evaluate the binding mode and stability of the Methylstat–JMJD2A complex. An in vitro BBB model was established to assess the ability of Methylstat to cross the BBB. Results: Methylstat significantly inhibited glioma cell proliferation in a dose-dependent manner without inducing apoptosis. It caused G1-phase arrest in U251 cells and G2-phase arrest in HOG cells. Mechanistically, methylstat downregulated cyclins and cyclin-dependent kinases via the p53/p21 pathway. Additionally, methylstat reduced the expression of JMJD2A and its downstream targets, including PDK1, AKT, and mTOR. Molecular docking studies and 200 ns MD simulations confirmed the stable binding of methylstat to the catalytic pocket of JMJD2A, effectively inhibiting its enzymatic activity. HPLC analysis confirmed that methylstat could penetrate the in vitro BBB model to varying extents. Conclusions: Methylstat is a promising small-molecule agent that effectively suppresses glioma cell growth by modulating key cell cycle regulators. Its ability to cross the BBB highlights its potential as a novel therapeutic strategy for GBM and other brain tumors.