(1) Objective: Cold atmospheric plasma (CAP) is a safe and effective alternative to radiotherapy for cancer treatment. Its anticancer effects are attributed to increased intracellular reactive oxygen species (ROS). Glutathione, a key antioxidant derived from glutamine, is critical for cell proliferation. This study investigated whether CAP-induced ROS elevation results from reduced glutamine–glutathione conversion and elucidates the underlying mechanisms. (2) Methods: Using esophageal squamous carcinoma cell models (Ec9706 and Eca109), we analyzed CAP’s effects on key enzymes in glutamine metabolism (Glutaminase 1 and γ-glutamylcysteine ligase) and proliferation-related genes (e.g., Retinoblastoma and Nuclear respiratory factor 2). Transcriptome analysis further explored molecular pathways involved in CAP-mediated anticancer effects. (3) Results: CAP reduced Glutaminase 1 and γ-glutamylcysteine ligase expression, leading to lower intracellular glutathione, higher ROS activity, and enhanced apoptosis. Transcriptome data confirmed CAP’s role in oxidation-reduction reactions and glutamine metabolism. (4) Conclusions: This study provides the first mechanistic insights into CAP’s anticancer effects by targeting glutamine metabolism. While based on in vitro assays, these findings guide the development of novel CAP therapies for currently incurable cancers.