Our main objective was to determine how distinct, natural thermal variation, representative of summer high temperature events, affected the thermal tolerance and physiology of wild juvenile Atlantic salmon, with the goal of informing conservation and management efforts. To this end, we acclimated fish to four thermal cycles, representative of real-world conditions but with differing minimum nighttime temperatures (16°C, 18°C, 20°C, 22°C) and diel temperature fluctuations (e.g. Δ5°C–Δ9°C). In each cycling condition, we measured critical thermal maximum (CTMax) as a proxy for acute thermal tolerance (Beitinger et al., 2000), recognizing that CTMax is influenced by acclimation temperature (e.g. Fangue et al., 2006). We also measured indicators of physiological (HSP70, ubiquitin) and energetic (lactate, glycogen) stress. We hypothesized that nighttime temperature throughout a thermal event dictates physiological limits. If supported, then we predicted that fish exposed to conditions with the highest nighttime temperature will have reduced acute thermal tolerance, and will experience enhanced metabolic and cellular stress compared to those exposed to cooler nighttime temperatures.