Athletic began playing in an improvised white kit, but in the 1902–03 season, the club's first official strip became half-blue, half-white shirts similar to those worn by Blackburn Rovers, which were donated by Juan Moser. Later, a young student from Bilbao named Juan Elorduy, who was spending Christmas 1909 in London, was charged by the club to buy 25 new shirts, but was unable to find enough. Waiting for the ship back to Bilbao and empty handed, Elorduy realised that the colours of the local team Southampton matched the colours of the City of Bilbao, and bought 50 shirts to take with him. Upon arriving in Bilbao, the club's directors decided almost immediately to change the team's strip to the new colours, and since 1910, Athletic Club have played in red and white stripes. Of the 50 shirts bought by Elorduy, half were then sent to Atlético Madrid, which had originally begun as a youth branch of Athletic. Before the switch from blue and white to red and white, only one other team wore red and white, Sporting de Gijón, since 1905.

Athletic were one of the last major clubs who did not have the logo of an official sponsor emblazoned on their kit. In the UEFA Cup and the Copa del Rey of 2004–05, the shirt sported the word "Euskadi" in green in exchange for hundreds of thousands of euros from the Basque Government (Red, white and green are the Basque colours). This policy was changed in 2008, when Athletic made a deal with the Biscay-based Petronor oil company to wear their logo in exchange for over €2 million. In 2011, Athletic revealed a new away kit that was inspired by the Basque flag. The Kutxabank logo now adorns the front of Athletic's kits.