In addition to forming N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSFs) with catechins, the Strecker degradation products of theanine can also combine with flavonol glycosides (FGs) to form EPSFGs during tea processing. In the present study, a novel adduct of theanine and rutin was identified and named as EPS-rutin. The effects of reaction temperature, time, initial reactant ratio, and water content on the generation of EPS-rutin in thermal reaction models were also analyzed. When the initial mass ratio of rutin to theanine was 1:5 and heating under 140 °C for 60 min, the content of EPS-rutin was the highest. Meanwhile, the content of EPS-rutin in green tea during processing was determined. Results showed EPS-rutin was formed during the first drying stage, and its level significantly increased after final drying. Furthermore, the bitterness and astringency thresholds of EPS-rutin were determined to be higher than those of rutin. This study further improved the understanding of the transformation pathways of theanine and polyphenols during tea processing, as well as contributed to exploring the flavor characteristics and health benefits of EPS-rutin.