Ambidextrous, the attacking midfielder was mainly known for his skills and shooting ability. A Spain international since 1993, he represented the nation at two World Cups, earning more than 40 caps and appearing in more than 400 official games for his only team.
Born in Portugalete, Biscay, Guerrero joined Athletic Bilbao as an eight-year-old, and subsequently progressed through the junior ranks of the club. Along with Aitor Karanka he was a member of the under-19 team that won a national double of cup and league in 1991–92, and also appeared and scored for the reserves in the second division during the same season.
In September 1992, aged 18, Guerrero made his senior debut for Athletic under Jupp Heynckes. He quickly made an impact and, although a midfielder, scored 28 La Liga goals in his first two seasons combined (65 in his first six). He was awarded the New Spanish Player of the Year by El País in 1993, and won the Spanish Footballer of the Year by both Don Balón and El País the following year. During the 1993–94 campaign, he scored a hat-trick against Albacete Balompié (4–1, home), adding four against Sporting de Gijón (7–0 also at the San Mamés Stadium).
The early promise Guerrero showed attracted interest from, among others, Real Madrid, FC Barcelona, Atlético Madrid, Juventus FC, S.S. Lazio and Manchester United. However, he remained loyal to Athletic and, in 1997, signed a ten-year contract which was the longest in the club's history, and also made him the team's highest-paid player.
In 1997–98, Guerrero netted eight times in 29 contests as the Basques finished second, leading to direct qualification for the UEFA Champions League. However, in the following years, his career went into decline and, in 2002, he was dropped by coach Luis Fernández, while still only 28; in his last four seasons combined he could only manage 57 games with just four goals but, despite his poor form, remained a fan favourite, and the decision to drop him caused some controversy.
On 11 July 2006, Guerrero announced his retirement as a player at an emotional press conference. He scored 116 goals in 430 competitive matches over the course of 14 seasons, including four in 17 appearances in various European competitions.
Immediately after retiring, Guerrero took charge of the Lezama youth ranks, leaving the post after two years.
On 27 January 1993, when still only 19, Guerrero made his debut for Spain in a friendly game with Mexico, in Las Palmas. He went on to play 41 games and score 13 goals, including one at the 1994 FIFA World Cup, and also represented the nation at the 1998 World Cup and UEFA Euro 1996.
Among Guerrero's personal international highlights were hat-tricks against Malta (3–0 in Attard, on 18 December 1996) and Cyprus (8–0, 8 September 1999). He played his last game for Spain in October 2000.
Between 1993 and 2006, Guerrero also played 11 times for the Basque Country autonomous team, scoring six goals. This included another hat-trick in 1997, against Yugoslavia.
Guerrero's younger brother, José Félix, was also a footballer and a midfielder. He also represented Athletic Bilbao, but only the reserves.
Julen also opened the Restaurante Julen Guerrero in Zamudio, and worked as an online columnist for Eurosport.Spain U21
UEFA European Under-21 Championship: Third-place 1994
La Liga Breakthrough Player of the Year: 1993
Best Spanish Player of the Year: 1994