Athletic Club "B", commonly known as Bilbao Athletic, is the reserve team of Athletic Club, a football club based in Bilbao, in the autonomous community of Basque Country in Spain. The team are playing in the Segunda Federación – Group 2. Founded in 1964, the team holds home matches at the small stadium attached to the club's training facility at Lezama, holding 3,250 spectators^[1] or occasionally at San Mamés Stadium, with its 53,500-seat capacity, for important fixtures.

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Athletic Bilbao B

Full name	Athletic Club "B"
Nickname(s)	Los Leones / Lehoiak (The Lions) Los Cachorros / Katxorroak (The Cubs)
Founded	1964; 60 years ago (1964)
Ground	Lezama (Field 2)
Capacity	3,250
Chairman.	Jon Uriarte
Manager	Carlos Gurpegui
League	Segunda Federación – Group 2
2022–23	Primera Federación – Group 2, 20th of 20 (relegated)
Home colours Away colours
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Reserve teams in Spain play in the same football pyramid as their senior team rather than a separate league. However, reserves cannot play in the same division as their senior team. Therefore, Bilbao Athletic are ineligible for promotion to La Liga. Reserve teams are also no longer permitted to enter the Copa del Rey. In addition, only under-23 players, or under-25 with a professional contract, can switch between senior and reserve teams. In recent years, most of Bilbao Athletic's players have been graduates from the club's youth setup ('cantera') via the feeder team, Basconia. As a result of Athletic's signing policy, only Basque players feature also for the reserve team.

The Bilbao Athletic name was first used in 1938 during the Spanish Civil War, when both La Liga and the Copa del Rey were suspended; most of the top Athletic players had joined the Euzkadi XI, a team put together at the suggestion of José Antonio Aguirre, the president of the Basque Country (and himself a former Athletic Bilbao footballer). Euzkadi went on tour to raise funds for the Basque cause, and also played in the Mexican domestic league. However, at home the Campeonato de Vizcaya had resumed in 1938. With their best players abroad with Euzkadi, Athletic could only field weakened sides and, to avoid possible shameful results damaging the club's reputation, chose to enter under the name Bilbao Athletic (the name was derived from the two clubs that merged in 1903 to become Athletic Bilbao – Bilbao Football Club and Athletic Club).^[2] Despite the low expectations, they still won the championship and entered the 1939 Copa del Generalísimo.

The name was revived in 1964, when Athletic Bilbao decided to establish a reserve team with Agustín Gaínza as coach. The new Bilbao Athletic initially played in local regional leagues before winning promotion to Tercera División in 1966, under Rafa Iriondo; in 1969 they first reached Segunda División.

In 1983–84, with José Ángel Iribar as coach, and an emerging Julio Salinas as striker, the reserves finished in second place, only trailing Castilla CF; both teams were ineligible for promotion, and Salinas won the Pichichi.

Bilbao Athletic dropped back down to the third level in 1996, but the main squad continued to be nurtured with several players who had spells with the reserves.

After 19 seasons in the Segunda División B, Bilbao Athletic returned to the second tier after defeating Cádiz CF, 3–1 on aggregate, in the promotion playoffs.^[3] However, in their campaign in the Segunda they were reliant on the same group, a core squad of 20-year-olds who had never played at such a high level before, and despite battling performances in most of their games, a pattern of narrow defeats led to the team finishing bottom and being relegated back down at the first attempt.^[4] Somewhat ironically, the promoted teams that season were CD Leganés whose squad included three players on loan from Athletic who would have been eligible to play for Bilbao Athletic that season, as well as the parent club's local rivals Alavés and Osasuna.

The team came close to another promotion in 2020–21, losing to Burgos after extra time in the final round of the promotion play-offs^[5] (they had lost in the opening round in 2018 and 2020).

• As a farm team

• As a reserve team

• 14 seasons in Segunda División
• 2 seasons in Primera Federación/Primera División RFEF
• 31 seasons in Segunda División B
• 1 season in Segunda Federación
• 10 seasons in Tercera División
• 2 seasons in Categorías Regionales

• Segunda División B:^{[lower-alpha 1]} 1982–83,^{[lower-alpha 2]} 1988–89^{[lower-alpha 2]}
• Tercera División:^{[lower-alpha 1]} 1966–67,^{[lower-alpha 3]} 1968–69^{[lower-alpha 4]}
• Biscay Championship: 1938–39

For big matches, they use San Mamés, the first team stadium.

• Javier Clemente
• Agustín Gaínza
• José Ángel Iribar
• Rafa Iriondo
• Ignacio Izagirre (284 matches)
• Iñaki Sáez (290 matches, record)^[8]
• José Ángel Ziganda (242 matches)

Note: This list includes players that have appeared in at least 100 top league games and/or have reached international status.

• CD Basconia (Athletic Bilbao's feeder club)