Born in Senegal, the 24-year-old moved to Pamplona as a child and began his career with Osasuna before joining Athletic in 2024.
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Boiro has made 45 appearances across La Liga, the Champions League and Europa League.
“I’m very excited and happy to be here!” said Boiro. “It’s a great club, and I can’t wait to get started.
“It was a hectic day, but we got through it calmly and everything turned out well, which is the important thing. Now I can’t wait to meet everyone and start contributing on the pitch.
“I like the challenge that’s in front of us here. West Ham is a Club with ambition and an objective to get promoted, and I think my mentality, my desire and my hard work will help my teammates and the Club achieve that.”