Solidago altissima L. (Asteraceae), a perennial plant native to North America, is considered one of the most invasive weeds in Asia and Europe. The successful invasion of S. altissima is possibly due to its allelopathic effect along with high seed productivity and strong vegetative propagation through rhizomes. Herein, to understand the invasion of S. altissima via the allelopathic effect, we isolated and characterized known and undescribed compounds from the underground parts of S. altissima and evaluated their contribution to the overall allelopathic activity of the plant. NMR spectroscopy and LC-MS analyses clarified the chemical structure of ten specialized metabolites including three undescribed compounds, i.e., (4Z, 8Z)-10-tigloyloxy matricaria lactone, (4Z, 8Z)-10-angeloyloxy matricaria lactone, and (2Z, 8Z)-10-methacryloyloxy matricaria ester. The evaluation of the content and allelopathic ability of each compound showed that cis-dehydromatricaria ester contributes to the allelopathic activities of the S. altissima extract.