At spring training in 2005, Chacín landed a spot in the Jays' starting rotation. He jumped out to a hot start in the American League, earning a record of 4–1 with 18 strikeouts and a 2.41 ERA over 32.2 innings through the month of April, and was selected the American League Rookie of the Month. In July, Chacín won five of his six starts, being named Rookie of the Month for the second time in the season. Chacín pitched and won the final game of the season for the Blue Jays against the Kansas City Royals with a solid pitching line of 7.2 innings, 7 hits allowed, 1 run, 1 walk, 3 strikeouts in the 7–2 Blue Jay win. Chacín finished the season with a record of 13–9 and 3.72 ERA in 34 starts. He collected 121 strikeouts in 203 innings. He finished fifth in Rookie of the Year voting, netting two first place votes and fourteen total points, and was named to the Topps All-Star Rookie Team along with fellow Blue Jay Russ Adams.