Lukaku joined his local team Rupel Boom at age five. After four seasons at Rupel Boom, Lukaku was discovered by scouts of Lierse, a Belgian Pro League club with an established youth academy. He played for Lierse from 2004 until 2006, scoring 121 goals in 68 matches.^[4] After Lierse were relegated from the Belgian Pro League, Anderlecht purchased 13 youth players from Lierse in the 2006 mid-season, including Lukaku. He played three more years as a youth player with Anderlecht, scoring 131 goals in 93 matches.^[5]

When Lukaku turned 16 on 13 May 2009, he signed a professional contract with Anderlecht lasting until 2012.^[6] Eleven days later, he made his Belgian First Division debut on 24 May 2009 in the championship play-off match against Standard Liège as a 69th-minute substitute for defender Víctor Bernárdez.^[7] Anderlecht lost the match 1–0. Lukaku became a regular member of Anderlecht's first team during the 2009–10 season, scoring his first goal at senior level against Zulte Waregem in the 89th minute after coming on as substitute for Kanu on 28 August 2009. "After scoring I dived into a sea of happiness", he told Berend Scholten from UEFA.com. "You think you are flying and can take on the whole world".^[8] He ended the season as the top goalscorer in the Pro League with 15 goals as Anderlecht won its 30th Belgian title.^[9] He also scored four goals during the club's run to the last 16 of the 2009–10 UEFA Europa League.^[9] During the 2010–11 season, Lukaku scored 20 goals in all competitions, but Anderlecht failed to retain their league title despite finishing top of the table during the regular season.^[10]

In August 2011, Lukaku joined Premier Leagueclub Chelsea on a five-year contract for a fee reported to be around €12 million (£10 million), rising to €20 million (£17 million) in add-ons.^[11]

Lukaku made his debut at Stamford Bridge in a 3–1 victory over Norwich City in the 83rd minute, coming on as a substitute for Fernando Torres.^[12]Lukaku made his first start for Chelsea in the League Cup against Fulham. Chelsea went on to win the match on penalties. He spent the majority of the season playing for the reserves and started his first Premier League match on 13 May 2012, against Blackburn Rovers, and turned in a man of the match performance, providing an assist for John Terry's opener.^[13] However, Lukaku stressed he was disappointed with his involvement at the end of his debut season, revealing that, after his side's UEFA Champions League win in the final on 19 May, he refused to hold the trophy, explaining "it wasn't me, but my team that won".^[14]

After speculation linking Lukaku with a loan move to Fulham,^[15] on 10 August 2012, he joined West Bromwich Albion on a season-long loan deal.^[16]He scored his first league goal eight days later, coming on as a substitute in the 77th minute in a 3–0 win against Liverpool.^[17] He made his full debut in a win against Reading at The Hawthorns, scoring the match's only goal.^[18] On 24 November, Lukaku came on as a 70th-minute substitute for Shane Long and netted a penalty and provided an assist to Marc-Antoine Fortuné, as West Brom defeated Sunderland 4–2 away at the Stadium of Light.^[19] The win proved to be West Brom's fourth consecutive win in the top flight for the first time since 1980.^[20] On 12 January 2013, Lukaku had his first multi-goal game in the Premier League, giving West Brom a 2–0 lead against Reading, before a late comeback gave the Berkshire club a 3–2 victory at the Madejski Stadium.^[21] In the face of claims he wanted to stay with West Brom for another year, Lukaku confirmed to the press he still wished to become a legend at Stamford Bridge.^[22] On 11 February, Lukaku came on as a substitute and scored his tenth Premier League goal of the season against Liverpool in a match that ended 2–0.^[23]

He scored his second brace of the campaign, scoring both goals for West Brom in their 2–1 home defeat of Sunderland on 23 February.^[24]On 9 March, in a league match against Swansea City, Lukaku scored the equalising goal before having a penalty kick saved; West Brom eventually won the game courtesy of a Jonathan de Guzmánown goal.^[25] On 19 May, coming on as a second-half substitute, Lukaku scored a remarkable second-half perfect hat-trick, as West Brom came from three goals down to draw 5–5 in the season's final home match against Manchester United.^[26] The match was Alex Ferguson's 1,500th and last match in charge of United, and the highest-scoring draw in the history of the Premier League.^[27] Despite being loaned out by the club, Lukaku outscored all of his Chelsea teammates in the Premier League that season, being the sixth-highest goal scorer of the 2012–13 season with 17 goals.^[28] Lukaku played two Premier League matches for Chelsea at the beginning of the 2013–14 season. He also came on as a substitute in the 2013 UEFA Super Cup, ultimately missing the deciding penalty in the shootout, as Manuel Neuer saved his shot and Chelsea lost to Bayern Munich.^[29]

On the final day of the summer 2013 transfer window, Lukaku joined Everton on a season-long loan.^[30] The striker made his début for the Toffees away to West Ham United on 21 September 2013, scoring the winning goal in a 3–2 victory for Everton. During the header, he had a collision with a West Ham defender and was taken off the pitch for some treatment. The physio had to tell him he had scored the winner.^[31] He scored twice on his home debut nine days later in a 3–2 win against Newcastle United, as well assisting Ross Barkley's goal.^[32] He then opened the scoring in a 3–1 defeat to Manchester City in the following match.^[33] He continued his impressive start at the club by netting the opening goal in a 2–0 win over Aston Villa, and then scored twice in the first Merseyside derby of the season, as Everton drew 3–3 against Liverpool, with Lukaku stating afterwards it was the best experience he had had in his short career.^[34][35]

In January 2014, Lukaku was named by The Guardian as one of the ten most promising young players in Europe,^[36] but later in the month was taken off on a stretcher with an ankle ligament injury after Gareth Barry slipped and slid into the striker, as he attempted to block Steven Gerrard's opening goal in the Merseyside derby against Liverpool.^[37][38] Lukaku made his return from injury against West Ham in March 2014, scoring the only goal of the match on the 81st minute after coming on as a second-half substitute.^[39]On 6 April, he scored one goal and assisted another as Everton beat Arsenal 3–0 at Goodison Park to record a sixth consecutive Premier League win.^[40] His final goal for the loan period came on the last day of the season as he scored the second in a 2–0 win over Hull City.^[41] Lukaku found the net 15 times in 31 league matches to help Everton to fifth place, with a club record of 72 Premier League points.^[42]

Lukaku signed a five-year contract with Everton in July 2014 for a then-club record fee of £28 million.^[43] He scored his first goal as a permanent player on 13 September, against his former club West Brom. Lukaku did not celebrate the goal, and was applauded by the West Brom fans for his sign of respect. On 19 February 2015, Lukaku scored his first hat-trick in a 4–1 win for Everton against Young Boys in the UEFA Europa League last 32; he struck with a header, a right-foot shot and a left-foot shot.^[44] He added another two goals in a 3–1 win in the second leg a week later.^[45] With eight goals, he was the tournament's joint highest scorer that season, alongside Alan of Red Bull Salzburg.^[46]

In the second game of the 2015–16 Premier League season, Lukaku scored a first-half double in Everton's 3–0 win at Southampton on 15 August 2015 with his first two shots on target. Before the match, he presented a t-shirt to a home fan whom he had accidentally struck with the ball during shooting practice.^[47] On 26 August, he scored another brace in a 5–3 extra-time win over League One side Barnsley in the League Cup.^[48] On 28 September, Lukaku scored twice and assisted the other against West Brom as he managed to guide his team from losing 2–0 to winning 2–3.^[49] He scored against Liverpool the following week in a 1–1 draw at Goodison Park.^[50] On 21 November, Lukaku scored twice in a 4–0 win over Aston Villa, becoming the fifth player under 23 years of age to score at least fifty Premier League goals, after Robbie Fowler, Michael Owen, Wayne Rooney and Cristiano Ronaldo.^[51] On 7 December, Lukaku scored a close-range goal in a 1–1 draw against Crystal Palace, his 50th in 100 appearances in all competitions for Everton.^[52] On 12 December, Lukaku became the first Everton player to score in six consecutive Premier League matches, and the first to score in seven consecutive matches in all competitions since Bob Latchford forty years previously, when he opened the scoring in the Toffees' 1–1 draw with Norwich City at Carrow Road.^[53] In his next match, a 3–2 defeat to Leicester City, Lukaku became the first Everton player since Dave Hickson in 1954 to score in eight consecutive matches.^[54]

On 6 February 2016, Lukaku scored his 20th goal of the season in a 3–0 victory at Stoke City, meaning he was the first Everton player since Graeme Sharp to score at least twenty goals in all competitions in consecutive seasons for Everton. The strike was also his 16th league goal of the season, equalling Premier League goal scoring records for Everton set by Tony Cottee and Andrei Kanchelskis in the mid-1990s.^[55] Lukaku followed this goal with another strike in a 2–0 victory at Bournemouth's Dean Court for his 21st goal of the season. This victory sent Everton into the quarter-finals of the FA Cup,^[56] and equalled the number of goals scored in all competitions by Yakubu for Everton in the 2007–08 season, the previous best in the Premier League era.^[57] On 1 March, Lukaku scored in a 3–1 victory against Aston Villa at Villa Park, his 17th league goal of the season, a Premier League-era record for Everton.^[58] The goal also meant Lukaku had equalled his previous best goal return in a league season, set during his loan spell at West Brom in the 2012–13 season.^[59]