Unlike in England, reserve teams in Spain play in the same football pyramid as their senior team rather than a separate league. However, reserve teams cannot play in the same division as their senior team. Therefore, the team is ineligible for promotion to La Liga, the division in which the main side plays. Reserve teams are also no longer permitted to enter the Copa del Rey.
Founded as San Sebastián Club de Fútbol, Real Sociedad's B team first reached the second division nine years later. In the 1961–62 campaign, it achieved its best ever classification in the category after finishing fifth; however, as the main squad was relegated in La Liga, it was ineligible for promotion, being relegated instead.
In 1980 Real Sociedad B promoted to the newly created Segunda División B, remaining in that category 17 consecutive seasons. Afterwards it fluctuated between divisions three and four, reaching the promotion playoffs of the former division in 1991 and 2006 but subsequently falling short.