The protein extraction from the blue-green microalgae Spirulina platensis was carried out using ultrasound-assisted extraction and response surface methodology (RSM) was used to optimise extraction conditions. Extraction yield, total phenolic content, antioxidant activity and in vitro protein digestibility of protein extracts were determined. A three factors Box-Behnken design (BBD) of experiments was employed at pH values of 7, 8 and 9; temperatures of 25, 35, and 45 degrees C; and for durations of 60, 90 and 120 minutes. Based on the RSM analysis, optimum extraction conditions (temperature 45 degrees C, pH 7.46 and time 120 min) were obtained for extraction yield (29.05%), total phenolic content (3.52 mg caffeic acid equivalent/g dw), antioxidant activity (11.32 mg Trolox equivalent/g dw) and in vitro protein digestibility (99.36%). We report the first evaluation of the in vitro protein digestibility of Spirulina platensis and find it to be over 90%. This value is higher than the in vitro protein digestibility values of proteins obtained from other algae and plant species, and, in particular, is greater than that of commercial soybean protein isolate.