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Pneumocystis jirovecii pneumonia (PJP) is a life-threatening opportunistic infection in immunocompromised patients. While classically seen in HIV/AIDS, it is increasingly recognised in those receiving lymphodepleting chemotherapy and radiation. Early identification and treatment are critical. A man in his 60s with glioblastoma multiforme on adjuvant temozolomide and cranial radiation presented with a 10-day history of progressive dyspnoea. Home pulse oximetry fell to 70% on room air. CT chest showed bilateral ground-glass opacities. Induced sputum PCR was positive for P. jirovecii; HIV serology was negative. He was started on high-dose intravenous trimethoprim–sulfamethoxazole and a prednisone taper, with de-escalation of broad-spectrum antibiotics. By hospital day 7, he remained on high-flow nasal cannula at 25 L/min fraction of inspired oxygen (FiO₂) 60%, maintaining a peripheral capillary oxygen saturation (SpO₂) ≥92%. He was speaking in full sentences and ambulating without desaturation. This case highlights the need for vigilance for PJP in solid-tumour patients receiving temozolomide and radiation, and the importance of early diagnosis and therapy.