Born in
San Sebastián,
Gipuzkoa, Prieto was a graduate of the
youth system at his hometown club
Real Sociedad, having joined from school aged 16 and spent time on loan with local amateurs
CD Hernani. His
La Liga bow took place 18 days later,
coming off the bench in the 1–0 home victory over
Navarrese neighbours
CA Osasuna; he would go on to collect a further eight substitute appearances – seven of them late in the game – during
that season. However, on 23 May of the following year, with the league's outcome already decided, he profited from his second start to score twice at
Real Madrid in a 4–1 win, one of the goals coming from the
penalty spot. Subsequently, Prieto was definitely promoted to the first squad and, in
2005–06, participated in all 38 games and netted nine times, six of them being penalties. He was also ever-present the
following campaign, but the
Basques dropped down to
Segunda División. Prieto played 35 matches in
2009–10, totalling exactly 3,000 minutes and contributing seven goals as Real Sociedad returned to the top flight after three years. On 6 January 2013, Prieto netted the first
hat-trick of his professional career, albeit in a 4–3 loss at Real Madrid: he scored once through a penalty and twice after one-on-one situations against
Iker Casillas. He added a brace on 22 February 2015 in another seven-goal thriller, but now in a 4–3 home defeat of
Sevilla FC; in the interim, he made seven appearances for the
Txuriurdin in the
2013–14 edition of the
UEFA Champions League. On 25 August 2017, Prieto played his 500th game for the Real Sociedad first team (only the fifth player to reach that milestone after
Juan Antonio Larrañaga,
Alberto Górriz,
Jesús María Zamora and
Luis Arconada) a few days prior to his 34th birthday, and marked the occasion with a goal in a 3–0 victory against Villarreal at the
Anoeta Stadium. He also received an ovation from supporters in the tenth minute, his shirt number, and a few weeks later he broke the
club record for most league appearances (including matches he played in the lower division, unlike the players he overtook). ==International career==