A drug-excipient interaction impurity associated with the degradation process of pramipexole was isolated. The impurity was detected during the stability study of pramipexole extended-release tablets. It was found at a relative retention time of 0.88 with respect to pramipexole, using the pramipexole gradient HPLC-UV detection method described in the USP. The structure of the impurity was identified and fully characterized using high resolution mass spectrometry, IR and NMR techniques, as presented herein. The degradation impurity was identified as (S)-N2-(methoxymethyl)-N6-propyl-4,5,6,7-tetrahydro-1,3-benzothiazole-2,6-diamine. The drug-excipient interaction mechanism of its formation was proposed. An efficient and straightforward synthetic approach was developed to prepare the degradation impurity to confirm its proposed degradation pathway and structure. © 2022 by the authors.