Born in Eibar, Gipuzkoa, Basque Country, Seguín joined Athletic Bilbao's youth setup in 2005, aged ten. He made his debut as a senior with the farm team in the 2012–13 campaign, in the fourth tier. On 26 May 2014, after scoring 11 goals for Basconia, Seguín was promoted to the reserves in Segunda División B. He appeared in 35 matches and scored three goals over the course of the season, as the B-side returned to Segunda División after a 19-year absence, via the play-offs. Seguín made his professional debut on 24 August 2015, starting in a 0–1 home loss against Girona FC. He scored his first goal in the second tier on 6 September, netting his team's second in a 3–1 home win against RCD Mallorca. In April 2016, having been involved in the majority of the season's fixtures and scored two further goals, Seguín suffered a rupture of the anterior cruciate ligament in his right knee, requiring a lengthy rehabilitation. He returned to the team nine months later at the end of January 2017, but only made seven appearances as a late substitute before the end of the campaign, having undergone minor keyhole surgery on the injury in March. Seguin featured in two of the first three games of the 2017–18 season, but reported injured in September and did not play again in the calendar year. Similar injuries were endured by Iker Undabarrena, Seguín's long-time teammate which hampered the development of both players and damaged their hopes of moving up to the senior squad; during Seguin's lengthy absences, fellow left winger Iñigo Córdoba was promoted instead. He continued his rehabilitation throughout 2018–19, but was unable to resume playing, instead watching on as two other teammates with ACL injuries – Oihan Sancet and Iñigo Baqué – underwent treatment and returned to action on schedule. In August 2019, he underwent stem-cell therapy on his knee and returned to training in November, though he was not registered to play in competitive fixtures for Bilbao Athletic. In January 2020, with the treatment apparently successful, he moved on loan to local club Barakaldo CF.
Personal life
Seguín's elder brother is also a footballer and a left winger by position; he too was groomed at Athletic Bilbao before leaving to play for other clubs in the lower leagues, serving as captain of third tier SD Amorebieta in 2017.
