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<i>Sambucus nigra</i> flowers, known as elderberry flowers (EBF), are a plant tissue rich in polyphenolic phytochemicals with important bioactivities. However, there are few studies dealing with the production of polyphenol-containing EBF extracts. The objective of the investigation presented herein was the development of a high-performance green extraction methodology, to generate EBF extracts enriched in polyphenolic substances, using an efficient deep eutectic solvent, combined with ultrasonication pretreatment. The DES was composed of L-lactic acid (hydrogen bond donor&#8212;HBD) and glycine (hydrogen bond acceptor&#8212;HBA) and, after an initial screening to properly regulate HBD/HBA ratio, the extraction was optimized by deploying response surface methodology. Under the optimized conditions, which were DES/water (85% <i>w</i>/<i>v</i>), liquid-to-solid ratio 60 mL g^&#8722;1, and stirring speed 200 rounds per minute, the extraction yield in total polyphenols amounted to 121.24 &#177; 8.77 mg gallic acid equivalents per g dry matter. The integration of ultrasonication prior to the batch stirred-tank extraction boosted polyphenol recovery of up to 174.73 &#177; 2.62 mg gallic acid equivalents per g dry matter. Liquid chromatography&#8722;mass spectrometry analysis showed that the richest EBF extract obtained was dominated by rutin, a di-p-coumaroylquic acid and chlorogenic acid.