He made his debut for Heerenveen, starting a match in the opening game of the season against FC Groningen and set up two goals, in a 3–3 draw. Dumfries set up three goals, with twice against ADO Den Haag and once against PSV Eindhoven on 26 August 2017 and 10 September 2017 respectively. Both of the matches led to Dutch media Algemeen Dagblad named him Team of the Week. He also awarded Eredivisie's Talent of the Month for August. During a match against PSV Eindhoven, Dumfries was subjected of the attention after Jürgen Locadia allegedly spat him, which was caught on camera, leading to an investigation from the KNVB committee. Eventually, Locadia was cleared by the committee after the player came forward with the investigation. Since making his debut for the club, he quickly established himself in the first team, playing in the right–back position. His performances impressed the Dutch media that Dumfries was named Team of the Week by Algemeen Dagblad at least four occasions in the 2017–18 season. On 9 December 2017, he scored his first goal for Heerenveen, scoring a header with a first equaliser, in an eventual 2–2 draw against VVV-Venlo. In a league match against AZ Alkmaar on 23 December 2017, Dumfries scored an own goal to give the opposition team an equaliser, which the club eventually lost 3–1. But he was able to make amends by scoring his second goal for Heerenveen, in a 2–1 win against his former club, Sparta Rotterdam on 26 January 2018. A week later on 6 February 2018, Dumfries scored his third goal for the club and setting up one of the goals, in a 3–2 loss against PEC Zwolle. After serving a one match suspension due to picking up five yellow cards this season, he returned to the starting line–up against Willem II and set up a goal, in a 2–0 win on 3 March 2018. In the first leg of the league playoffs for the place in the UEFA Europa League spot against FC Utrecht, Dumfries scored his fourth goal of the season, in a 4–3 win. However, in the return leg, FC Utrecht managed to turn the deficit with the results by winning 2–1, resulting the opposition team going through, thanks to away goal rule. At the end of the 2017–18 season, he went on to make thirty–seven appearances and scoring four times in all competitions.
