The objective of this study was to explore the changes in leaf nitrogen (N) and phosphorus (P) content, physiological processes, growth, and resource-use efficiency in a rapeseed cultivar under drought and (or) high temperatures. The two-wk-old plants were grown under controlled growth chamber conditions and subjected to individual or combined water (well-irrigated, 88% ± 4% field capacity vs. drought, 46% ± 5% field capacity) and temperature (control, 23 °C/17 °C vs. high temperatures, 26 °C/25 °C) regimes for 10 d. The measured response parameters were [N], [P], and their total content in leaves, photosynthetic rate (A_sat), stomatal conductance (g_s), intercellular CO₂ concentration (C_i), mitochondrial respiration (R), intrinsic water use efficiency (WUE_i), photosynthetic N use efficiency (PNUE), relative growth rate of the root (RGR_r) and shoot (RGR_s), leaf area, and dry matter accumulation in the plant. Drought significantly decreased [N], [P], and their total content in leaves, A_sat, g_s, C_i, RGR_r, RGR_s, leaf area, dry matter accumulation in the root, shoot, and whole plant, and PNUE, but significantly increased R and WUE_i. Drought-induced reduction in growth or A_sat was mainly attributed to a decreased C_i due to stomatal closure, while reduction in g_s and leaf area appeared to be a drought-adaptive mechanism. High temperature stress alone had no negative impact on physiological and growth parameters, indicating an enhanced thermal stability of the cultivar, which was diminished at combined drought and high temperature stresses. We therefore conclude that the thermal stability of the cultivar in terms of growth was compromised under simultaneous occurrence of drought and high temperature stresses.