The Spanish side will travel to England and play Ralph Hasenhuttl's team on August 7th.
Saints manager Hasenhuttl will hope to use the Bilbao game to finalize his team for the opening league game of the season against Everton at Goodison Park the following week.
Athletic recently won the Spanish Super Cup, while they reached the final of the Copa Del Rey as well.
The two sides last met in a friendly in 2016, when a Shane Long goal settled the outcome.
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