After seeing off Mensur Suljović 3–1 in the first round of the 2015 World Championship, Smith beat Brendan Dolan 4–2 after having been 2–0 down, to set up a third round tie with fellow St. Helens player Stephen Bunting. Despite taking out checkouts of 170 and 132, Smith was 3–0 down, before finding his scoring game to restrict Bunting to one leg as he closed the gap to 3–2. Ultimately his slow start to the match proved costly as he lost the next set to be knocked out 4–2 in a game that saw each player average 102 and hit eight 180s apiece. Smith won the fifth UK Open Qualifier by beating Adrian Lewis 6–5. A rematch with Bunting followed in the third round of the UK Open, with Smith missing two match darts in the deciding leg to be beaten 9–8. He claimed the second European Tour title of his career by winning the International Darts Open with a 6–3 victory over Benito van de Pas in the final. Smith suffered a 10–4 defeat to Gerwyn Price in the first round of the World Matchplay, but retained his European Darts Trophy title by seeing off Michael van Gerwen in the final for the second year in a row. At the World Grand Prix he punished two missed match darts from Price to beat him 2–1, but Smith then lost from 2–0 up in sets against Jamie Lewis in the second round. Wins over Wayne Jones, Lewis and Andy Fordham saw Smith top his group at the Grand Slam and he then averaged 104.59 to beat Dave Chisnall 10–7 and reach the quarter-finals. Smith's first major event semi-final soon followed as he defeated Lewis 16–11, but he was heavily beaten 16–6 by Van Gerwen.