Using a potted experiment, we studied the effect of slow-release nitric oxide (NO) on the physiological characteristics of cotton seedlings subjected to salt stress (585 mg/kg NaCl). Sodium nitroprusside (SNP, an NO donor, 2.62 mg) was applied either directly to the soil, via slow release methods (slow-release bags, slow-release capsules, or slow-release particles), or via foliar application. NaCl decreased plant weight and chlorophyll content and increased electrolyte leakage and the contents of proline, ascorbic acid (ASA), and Na. NaCl also induced oxidative stress, as indicated by elevated levels of lipid peroxidation and ROS production. Foliar spray of SNP (0.09 mM) enhanced plant growth, promoted ion absorption and transport, and increased enzyme activity. Slow-released NO increased the levels of chlorophyll, ASA, and proline; promoted the uptake of K, Ca and Mg; and decreased Na. Moreover, increases in SOD and CAT activity were demonstrated to counter oxidative stress. However, different methods of SNP application have different effects on salt-tolerance. Foliar application was optimal. Slow-release NO, especially slow-release particles, was better able to alleviate NaCl toxicity compared with direct application of NO to the soil.