Born in Barrio del Progreso, Murcia, Martínez began his footballing career with local side C.F.S. El Progreso and later joined Real Murcia. In 2013, he signed for Valencia. Initially a member of the club's academy Cadete A team, Martínez was soon admitted to their reserve side, Valencia Mestalla. He made his senior debut in the Segunda División B at the age of 16, coming on as a substitute and scoring in a match against AE Prat on 8 December 2013. In three seasons at Valencia Mestalla he played 16 games and scored two goals, and made a further eight appearances, a goal and three assists for the club's under-19 squad in the UEFA Youth League.
West Ham United
2016–17: Signing and Oxford loan
In April 2016, Martínez agreed a three-year deal to join English club West Ham United on 1 July 2016 for a fee of around £2.4 million. He was given permission to train with his new teammates by Valencia to help him settle in England and would initially link up with West Ham United's development squad. Martínez was given the number 29 shirt for the 2016–17 season. He marked his Premier League 2 debut with a goal against Stoke City, in a 3–0 home win. After scoring seven goals in as many games for West Ham United U23 during the first half of the season, he suffered two injury set-backs; the first left him sidelined for six weeks with the player due to return to action on 21 November, but then suffered another injury. His prolific goal-scoring feats continued into January and before long he had racked up a total of 12 goals from 11 games. On 23 January 2017, he was sent out on loanfor the first time in his career, to League One side Oxford United until the end of the 2016–17 season. The player was handed the number 7 shirt by the club. On 28 January, Martínez came on as a substitute to score his first goal for the U's in a 3–0 home win over Newcastle United in the fourth round of the FA Cup. He made his league debut on 5 February in an away fixture against local rivals Swindon Town, a game which Oxford won 2–1, and scored his first league goal, a late consolation goal in a 3–2 home defeat at the hands of league leaders Sheffield United, on 7 March.
2017–18: Debut and Valladolid loan
On 7 January 2018, Martínez made his full West Ham debut in an FA Cup third round tie at Shrewsbury Town, replacing Javier Hernández after 71 minutes in a 0–0 draw. On 1 February, he made a deadline day loan switch to Spanish Segunda División club Real Valladolid for the remainder of the 2017–18 season. He contributed with one goal in ten appearances, as his side achieved promotion to La Liga in the play-offs.
2018–19: Rayo Majadahonda and Lugo loans
On 13 August 2018, Martínez joined CF Rayo Majadahonda still in Spain's second division, on loan for the season. He made his debut six days later on the opening weekend of the season against Real Zaragoza, Majadahonda's first game at this level. On as a second-half substitute for Enzo Fernández, he scored the club's first goal in the Segunda División, albeit in a 2–1 defeat. On 18 January 2019, Martínez rescinded his loan deal with Majadahonda and moved to CD Lugo of the same league, on loan until June. On his debut eight days later, he scored the opener in a 3–2 home win against his former club. He was released by West Ham on 1 July 2019, when his contract expired.
Famalicão
On 22 July 2019, Martínez joined F.C. Famalicão on a three-year contract. He made his debut on 3 August in the first round of the Taça da Liga, as a 59th-minute substitute for Pedro Gonçalves in a 2–0 home loss to S.C. Covilhã. A week later, in his team's first Primeira Liga match for a quarter of a century, he scored the first goal of the season in a 2–0 win at C.D. Santa Clara. Martínez scored four goals in five games as Famalicão reached the semi-finals of the Taça de Portugal for the first time since 1946. All came in consecutive games, including one in each leg of a 4–3 aggregate elimination by S.L. Benfica in February 2020.
