Background: Andrographis echioides (L.) Nees is an annual herb mainly distributed in India and Sri Lanka. In traditional medicine system, the plant is used for treating various ailments such as fevers, skin diseases, stomach ache, toothache, snake bite, and eczema. The whole plant of A. echioides was reported as the rich source of flavonoids. Ultrasound-assisted extraction (UAE) is an effective extraction method used for secondary metabolite extraction from various plant materials over conventional methods. Today, the response surface methodology (RSM) is a successful statistical tool used to optimize the various extraction conditions of the secondary metabolite from various sources. Objective: The objective of this study is to optimize the UAE conditions such as ethanol concentration (50%–100%), solvent-to-solid ratio (10–50 mL/g), and sonication time (20–60 min) for the extraction of flavonoids and anti-oxidant capacity (AOC) from A. echioides (L.) Nees whole plant (AEWP) using the RSM strategy with Box–Behnken design (BBD). Materials and Methods: UAE conditions, i.e. ethanol concentration, solvent-to-solid ratio, and sonication time, were optimized with the corresponding responses of flavonoid yield and %DPPHAOCand %ABTSAOCby RSM. The effect of ultrasound on plant material was analyzed using Scanning electron microscope (SEM). The efficiency of the optimized extract was analyzed using Fourier-transformed infrared spectroscopy (FTIR) and liquid chromatography-mass spectra (LC-MS). Results: The BBD provided adequate mathematical models that accurately describe the behavior of the technique and help to predict the flavonoid yield, %DPPHAOCand %ABTSAOCfrom AEWP. The optimized UAE conditions were 77% of ethanol concentration, 35 mL/g of solvent-to-solid ratio, and 41 min of sonication time. Under these extraction conditions, UAE would obtain a maximum of 10.91 ± 0.04 mg CE/g for flavonoid yield, 87.36 ± 0.06% for %DPPHAOC, and 85.14 ± 0.03% for %ABTSAOC.The obtained experimental results of all the responses are in good agreement with the predicted values. SEM analysis explores the effect of UAE compared with the conventional extraction. The FTIR and LC-MS analysis revealed that the optimized extract of AEWP is rich in flavonoids; apart from the known flavonoids, five new flavonoids were identified from this optimization study. Conclusion: The study confirmed that UAE was the effective extraction method for the extraction of flavonoids from AEWP with ethanol as a solvent of choice with a low solvent usage in a reasonable time