The grass Phragmites australis has historically been restricted to the relatively benign upper border of coastal marshes, but over the past century, and particularly in recent decades, it has been spreading aggressively in New England throughout brackish and salt marshes with high soil salinities that are physiologically stressful to the plant. Here I tested the hypothesis that variations in climatic conditions, particularly increased precipitation during the 1997-98 El Niño event, buffer harsh abiotic conditions and enhance the performance of this nuisance species. I monitored the growth and reproductive output of P. australis in the year before, during, and after the 1997-98 El Niño in coastal brackish marshes of southern New England, USA. During the El Niño year, P. australis produced on average 30% more shoots, which were 25% taller, and yielded an order of magnitude more inflorescences than in the other 2 years. Soil porewater salinities were negatively related to precipitation during the 3 years of the study, and the growing season during the El Niño year was one of the wettest of the past century. Consequently, increased precipitation during El Niño may facilitate the spread of less salt-tolerant nuisance and invasive species throughout brackish and salt marshes.