•Nano-K maximized salt-stressed squash plant productivity and yield quality.•Nano-K catalyzed more antioxidant activities, attenuating damage of oxidative stress markers.•Nano-K efficiently improved photosynthesis and plant integrity under salt stress.•Nano-K efficiently restored the nutrients of salt-stressed squash plants.•Nano-K efficiently improved antioxidant enzymes and their gene expression in salt-stressed squash plants. To explore the reinforcing effects of nano-potassium fertilizer (nano-K) on growth, yield and quality, physio-biochemical responses, antioxidant defense systems, antioxidant gene expressions, and nutrients of salt-stressed squash plants, field experiments were performed using normal (EC = 2.34 dS m−1) and saline (EC = 9.38 dS m−1) soils during the 2021 and 2022 seasons. The traditional recommended K at full dose (TRK100) or half dose (TRK50) was used in both soils. Salinity and/or TRK50 significantly decreased photosynthetic pigment contents (by 8.7−22.5%), photosynthetic efficiency (by 6.0−10.9%), relative water content (RWC by 5.0−5.1%), membrane stability index (MSI by 7.2−7.4%), nutrient contents (by 28.4−48.2%), K+/Na+ ratio (by 66.4−67.1%), which contributed to the decrease in growth (by 12.2−25.8%) and yield traits (by 4.2−28.5%), and fruit quality (by 8.5−19.7%) due to an increment of oxidative stress biomarker (O2•‒ by 32.4−52.9% and H2O2 30.4−57.1%) levels, electrolyte leakage (EL by 23.3−24.5%), malondialdehyde (MDA by 71.4−77.6%), and Na+ (by 55.5−56.9%). Under stress in both soils, foliar-applied 0.50 g nano-K L−1 suppressed the levels of O2•‒, H2O2, MDA, EL, and Na+, while noticeably increased photosynthetic pigment contents, photosynthetic efficiency, RWC, MSI, nutrient contents, K+/Na+ ratio, which were positively reflected in growth and yield traits, and fruit quality due to increased osmoprotectant and low-molecular-weight antioxidant contents, antioxidant enzyme activities and enzymatic gene expressions. Therefore, our findings recommend using nano-K as an effective strategy to promote antioxidant and photosynthetic machineries, minimize oxidative stress biomarkers and Na+ levels, boost tolerance to salt stress, and improve squash yield and yield quality under salt stress. Shows the reinforcing effect of nano-K on the antioxidant defense system reflecting the increased yield and quality of salt-stressed squash plants Display omitted