On 1 January 2010, Wirth signed a contract with Unión Española, after being released by Colo-Colo. Rainer officially debuted on 16 July in a 2–1 home loss against Unión San Felipe, recently in the second part of the tournament after of the FIFA World Cup. On 5 September, Wirth again started for Unión in a 3–2 win over Everton, and in a surprise move five days later, was selected by the coach José Luis Sierra for play against Universidad Católica. He again was selected by Sierra for play in the last games of the tournament and in the Copa Libertadores playoffs ahead of Luis Marín due to Wirth's good performance. He also won the Libertadores playoffs title, after the defeat of Audax Italiano in the final.