Robert Thornton (born 17 July 1967) is a Scottish professional darts player who formerly competed in British Darts Organisation (BDO) and Professional Darts Corporation (PDC) events. In the BDO he won the 2007 Winmau World Masters and 2008 Dutch Open. He switched to the PDC in 2008 and won 2 PDC major titles; the UK Open in 2012, and World Grand Prix in 2015. Additionally, Thornton won 11 titles on the PDC Pro Tour.

After losing his PDC Tour Card, Thornton won the World Seniors Darts (WSD) World Championship twice. He also won the 2022 World Seniors Matchplay.

Thornton returned to playing darts in 2002, having given up the game for two decades to raise his children. He qualified for the BDO World Championship for the first time in 2005. He beat Martin Atkins and Tony West before losing to Darryl Fitton in the quarter-finals. He then failed to qualify for the 2006 and 2007 events.

Thornton's first professional title came in the Central Scotland Open in April 2007, beating Mike Veitch in the final. He also won the Scottish Masters by defeating Ross Montgomery. He failed to make it through the qualifying event for the 2008 World Championship, but did make it to the televised stages of the 2007 Winmau World Masters in the same weekend. Thornton then beat Co Stompé, BDO world champion Martin Adams, and Atkins to reach the final. He managed to avenge his defeat by Fitton at the 2005 Lakeside World Championship to claim his first major title by 7 sets to 5 and win the £25,000 first prize and also a place in the 2008 World Championship.

His appearance at the World Championship was only his second visit to Lakeside and he matched his previous appearance by reaching the quarter-finals. Having beaten John Walton and Tony O'Shea, he lost to defending champion Adams. Thornton's good form continued into 2008 when he won the Dutch Open in February and reached the last eight of the Scottish Open a week later to achieve a career high of third in the WDF world rankings and was top of the BDO rankings.

Thornton qualified for the 2008 UK Open via the Pub qualifiers and then went on to switch to the PDC after withdrawing from the Scottish team for the Europe Cup, stating that his first goal was to get into the top 16 of the PDC Order of Merit which would guarantee his place in all the major tournaments. At the UK Open he lost 9–6 to Alan Tabern in the last 32. Thornton qualified for the Las Vegas Desert Classic losing again in the last 32 this time 6–5 against Adrian Lewis, but went one better at the World Grand Prix when he beat Peter Manley 2–1 in sets in the first round before being defeated 3–2 by Andy Hamilton in the last 16. His good start to his career in the PDC continued at the European Championship where he saw off John Part, Mensur Suljović and Ronnie Baxter to advance to the semi-finals for the first time and a meeting with Phil Taylor. Thornton averaged 102.12 in the match but Taylor averaged 113.33 as he won 11–7. Thornton's World Masters title from last year saw him qualify for the Grand Slam of Darts and he advanced from Group E with wins over Gary Mawson and Kirk Shepherd, before being edged out 10–8 by Terry Jenkins in the last 16. Away from the television cameras on the PDC Pro Tour he won the Australian Open Players Championship with a 3–1 success against Paul Nicholson. He was a losing semi-finalist three times and a quarter-finalist four times in the other events to finish as the highest non-qualified player on the Pro Tour Order of Merit for the 2009 PDC World Championship.

At the World Championship, he won 3–1 against Wayne Jones, but went out in the second round with a 4–3 defeat to world number eight Wayne Mardle. However, this performance guaranteed Thornton a spot in the top 32 of the PDC Order of Merit – just eight months after his defection from the BDO. He reached his first PDC final at the Players Championship Finals. Seeded 14 in the tournament based on the previous year's floor events, Thornton edged Mark Dudbridge in the first round on a final-leg decider. This gave him a second round tie with world number 10 and third seed Alan Tabern, in which Thornton dominated 6–2 after Tabern had missed a dart for a 3–1 lead. He then played a resurgent Dennis Priestley – who had earlier been on the brink of defeat, losing 5–0 to John Part before rattling off six consecutive legs to win the match – in the quarter-finals. Those heroics seemed to fatigue Priestley as he slumped to a 9–5 defeat to Thornton, which earned him a semi-final place against world number two James Wade. In a very tight semi-final, Thornton won four of the last five legs to earn a 13–10 victory and a spot in the final against Phil Taylor, who had only dropped eight legs in the tournament to that point. With the scores level at 5–5 early on, Taylor came out on top 16–9, however Thornton earned £25,000 for making the final, propelling him up the rankings. Thornton became only the third player to win both events of a PDC Pro Tour weekend in Irvine, North Ayrshire, when he beat Dennis Priestley to win both the Scottish Players Championship and the Scottish Regional qualifier for the UK Open. He made his Premier League debut in Aberdeen in an exhibition match against James Wade. Thornton lost 7–2. By this time, Thornton had reached number 19 in the PDC Order of Merit. He lost 9–2 to Adrian Lewis in the third round of the UK Open and 10–4 to Taylor in the first round of the World Matchplay. Thornton saw off Wes Newton 6–4 in the opening round of the European Championship, but lost to Taylor once more as he was whitewashed 9–0, with Thornton's average of 83.24 being almost 30 points lower than his opponents. Thornton topped Group H at the Grand Slam of Darts and then edged past Darin Young in the last 16 10–9, but was knocked out 16–9 in the quarter-finals by Terry Jenkins.

Thornton defeated two qualifiers in Christian Perez and Jyhan Artut at the 2010 World Championship, before Taylor was once again the victor when the two met in the last 16 this time winning by four sets to one.