Cucumber (Cucumis sativusL.) is widely valued for its nutritional and nutraceutical attributes; however, its postharvest quality and proximate composition under varying agronomic practices in tropical Africa remain insufficiently documented. This study evaluated the effects of variety, plant spacing, and NPK (15-15-15) fertilizer rate on fruit firmness, colour (CIE a*), proximate composition, and vitamin content of cucumber grown in southwest Nigeria. Field experiments were conducted at the Federal University of Agriculture, Abeokuta (7°15′N, 3°25′E; 144 m a.s.l.) during the early and late seasons of 2019 and 2020. Treatments comprised two varieties (CU 999 and Monalisa), three spacings (75 × 25, 75 × 50, and 75 × 75 cm), and four fertilizer rates (0, 100, 200, and 300 kg ha⁻¹), arranged in a split-split plot randomized complete block design with three replications. Fruit texture, colour, vitamins, and proximate components were determined using standard analytical methods. Data were analysed using analysis of variance, and means were separated using LSD at P ≤ 0.05. CU 999 produced significantly firmer fruits (up to 1.35 kg) than Monalisa (1.15 kg), while Monalisa exhibited deeper green colour. Application of 300 kg ha⁻¹ NPK significantly increased vitamin A (35.93 μg 100 g⁻¹), vitamin C (3.41 mg 100 g⁻¹), vitamin K, dry matter, and carbohydrate content. Wider spacing (75 × 75 cm) enhanced green colour intensity, vitamin K, and dry matter. Significant interactions among factors were observed, with the 75 × 75 cm and 300 kg ha⁻¹ combination generally optimizing fruit quality.