In 2015, Ertz missed almost half of the NWSL season due to commitments with the United States women's national team at the FIFA Women's World Cup in Canada. She returned to the Red Stars on July 22 during a match against the Boston Breakers, which ended in 2–1 victory for the Red Stars. She made 11 appearances for the Red Stars during the 2015 season, playing 990 minutes. The Red Stars finished second in the regular season with an 8–3–9 record and advanced to the playoffs. The Red Stars faced FC Kansas City in the semi-finals of the playoffs on September 13. Ertz started in the match, which ended in a 0–3 defeat for the Red Stars and they were eliminated from the playoffs. Following the 2015 season, Ertz was named to the NWSL Best XI.