Nonsteroidal anti-inflammatory drugs (NSAIDs), which are among the most widely used analgesics in the world, cause gastrointestinal inflammation that is potentially life-threatening. Although inhibitors of endocannabinoid catabolic enzymes protect against gastropathy in fasted NSAID-treated mice, the gastroprotective effects of Δ9-tetrahydrocannabinol (THC), the primary psychoactive component of marijuana, have yet to be investigated. Male C57BL/6J mice were fasted, administered vehicle or Δ9-THC (.01–50 mg/kg; oral or intraperitoneal), and then treated with the NSAID diclofenac sodium (100 mg/kg, p.o.) to induce gastric lesions. In separate groups of mice, the cannabimimetic behavioral effects of Δ9-THC given via each route of administration were compared using a battery of tests, consisting of assessment of locomotor activity, nociception in the tail withdrawal test, catalepsy in the bar test, and hypothermia. Δ9-THC dose-dependently attenuated diclofenac-induced gastric hemorrhagic streaks through both p.o. and i.p. routes of administration (ED50 (95% confidence interval)=0.64 (0.26–1.55) mg/kg and 0.06 (0.01–0.34) mg/kg, respectively). Δ9-THC given i.p. was 2–3 orders of magnitude more potent in reducing diclofenac-induced gastric ulcers than in producing locomotor immobility, antinociception, hypothermia, and catalepsy, while the potency of ratio of p.o. Δ9-THC between each behavior measure was 7–18. These data indicate that the phytocannabinoid Δ9-THC protects against diclofenac-induced gastric inflammatory tissue damage at doses insufficient to cause common cannabinoid side effects.