In our previous studies, we found that a single ultralow dose of tetrahydrocannabinol (THC; 0.002 mg/kg, three to four orders of magnitude lower than the conventional doses) protects the brain from different insults that cause cognitive deficits. Because various insults may trigger a neuroinflammatory response that leads to secondary damage to the brain, the current study tested whether this extremely low dose of THC could protect the brain from inflammation‐induced cognitive deficits. Mice received a single injection of THC (0.002 mg/kg) 48 hr before or 1–7 days after treatment with lipopolysccharide (LPS; 10 mg/kg) and were examined with the object recognition test 3 weeks later. LPS caused long‐lasting cognitive deficits, whereas the application of THC before or after LPS protected the mice from this LPS‐induced damage. The protective effect of THC was blocked by the cannabinoid (CB) 1 receptor antagonist SR14176A but not by the CB2 receptor antagonist SR141528 and was mimicked by the CB1 agonist ACEA but not by the CB2 agonist HU308. The protective effect of THC was also blocked by pretreatment with GW9662, indicating the involvement of peroxisome proliferator‐activated receptor‐γ. Biochemical examination of the brain revealed a long‐term (at least 7 weeks) elevation of the prostaglandin‐producing enzyme cyclooxygenase‐2 in the hippocampus and in the frontal cortex following the injection of LPS. Pretreatment with the extremely low dose of THC tended to attenuate this elevation. Our results suggest that an ultralow dose of THC that lacks any psychotrophic activity protects the brain from neuroinflammation‐induced cognitive damage and might be used as an effective drug for the treatment of neuroinflammatory conditions, including neurodegenerative diseases. © 2014 Wiley Periodicals, Inc.