Cucumber Fusarium wilt, induced by Fusarium oxysporum f. sp. cucumerinum (FOC), causes severe losses in cucumber yield and quality. Nitrogen (N), as the most important mineral nutrient for plants, plays a critical role in plant–pathogen interactions. Hydroponic assays were conducted to investigate the effects of different N forms (NH₄⁺ vs. NO₃^‒) and supply levels (low, 1 mM; high, 5 mM) on cucumber Fusarium wilt. The NO₃^‒-fed cucumber plants were more tolerant to Fusarium wilt compared with NH₄⁺-fed plants, and accompanied by lower leaf temperature after FOC infection. The disease index decreased as the NO₃^‒ supply increased but increased with the NH₄⁺ level supplied. Although the FOC grew better under high NO₃⁻ in vitro, FOC colonization and fusaric acid (FA) production decreased in cucumber plants under high NO₃⁻ supply, associated with lower leaf membrane injury. There was a positive correlation between the FA content and the FOC number or relative membrane injury. After the exogenous application of FA, less FA accumulated in the leaves under NO₃⁻ feeding, accompanied with a lower leaf membrane injury. In conclusion, higher NO₃⁻ supply protected cucumber plants against Fusarium wilt by suppressing FOC colonization and FA production in plants, and increasing the plant tolerance to FA.