Coat a large saucepan with cooking spray; place over medium-high heat until hot. Add mushrooms, onion, and garlic; saute until tender. Stir in water, salt, and pepper; bring to a boil. Remove saucepan from heat.
Stir in couscous; cover and let stand 5 minutes or until couscous is tender and liquid is absorbed. Fluff couscous with a fork.