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A response surface methodology was used to analyze the flow rate, power, and time factors of plasma surface treatment. Surface free energy (SFE) of treated glass fiber-reinforced composites showed a strong quadratic dependence on flow rate, power, and time, with significant interaction between time and power. Optimized factors predicted a maximum SFE of 78.63 mN/m, which matched well with the measured value of 77.42 mN/m, accounting for 2.46 times increase in SFE against untreated case. Moreover, with plasma treatment, the SFE’s polar component became dominant (99%) as also confirmed with FTIR spectroscopy. Fracture toughness testing of fresh and aged adhesive joints proved a more stable interface for plasma-treated specimens due to the covalent bonds facilitated by the functional groups formed during the treatment. Consequently, the fracture toughness of the plasma-treated specimens did not drop after seawater immersion, while that for the untreated and sand-treated specimens showed about a 15% drop.