At the start of the 2017–18 season, Sakai continued to establish himself in the starting eleven for the side, playing in the right–back position despite finding himself in a competition with Bouna Sarr over the right–back position. He stated in an interview with France Football that he has settled in France, having suffered difficulties in his previous season. His performance in a 2–0 win over Toulouse on 24 September 2017 led him to be named Team of the Week. Following good performance for the side this season so far, Sakai signed a four–year contract with the club, keeping him until 2021, on 29 September 2017. Having played in the right–back position in the first half of the season, Sakai switched to playing in the left–back position following the departure of Patrice Evra and injury of Jordan Amavi. He spent the next six matches, playing in the left–back position before returning to his original position. During a 3–2 loss against Lyon on 18 March 2018, he suffered an injury in the first–half and was substituted as a result, leading him to be sidelined between 7–10 days. He made his return from injury on 5 April 2018 in the quarter–final of the UEFA Europa League first leg, in a 1–0 loss to RB Leipzig. On 12 April 2018 (which was his 28th birthday), Sakai scored his first UEFA Europa League goal in the quarter–final return leg, in a 5–2 win over RB Leipzig to progress into the next round and a 5–3 aggregate victory. Nine days later after the win, he sustained an injury during a 5–1 win over Lille and was substituted in the first half as a result; which he was sidelined for three weeks. Despite recovering from his injury in mid–May, Sakai, however, was featured on the substitute bench throughout 90 minutes, as Marseille lost 3–0 to Atlético Madrid in the UEFA Europa League Final. At the end of the 2017–18 season, Sakai finished the season, making 50 appearances and scoring twice in all competitions.