[5] On 24 April 2016, Feyenoord beat FC Utrecht 2–1 in the final to win the KNVB Cup, but Nieuwkoop did not play in the competition.

[6] Nieuwkoop made his European debut on 15 September 2016, getting subbed on for Rick Karsdorp in injury-time of a 1–0 win against Manchester United in the UEFA Europa League group stage.

[9] He missed the first part of the new season, including the won Johan Cruyff Shield against Vitesse,[10] due to an injury, gaining competition for the right-back position from Kevin Diks, Sofyan Amrabat and Jeremiah St.

On 30 April 2021, it was announced that Nieuwkoop had signed a three-year contract with Union Saint-Gilloise, recently promoted to the Belgian Pro League as champions of the First Division B, on a free transfer.

On 2 August 2022, Nieuwkoop made his European debut for Union Saint-Gilloise as the club beat Rangers 2–0 in the UEFA Champions League third qualifying round.

[16] Three days later, he made his return debut, starting in Feyenoord's opening match of the 2023–24 Eredivisie season, a 0–0 home draw against Fortuna Sittard.

[17] Nieuwkoop's return to De Kuip did not unfold as anticipated, after being sent off in the 25th minute due to stepping on Iñigo Córdoba's ankle, for which he got suspended for two league games.

[20] On 18 February 2024, he was subbed on for Thomas Beelen at half-time to play his hundredth official game for Feyenoord, a 1–0 league win against RKC Waalwijk.

[22] Nieuwkoop made his debut for the Netherlands under-17 on 25 October 2012, replacing Stijn Spierings during a goalless draw against Lithuania in the 2013 UEFA Euro qualifiers.

[26] Nieuwkoop played his only game for the Netherlands under-21 on 22 March 2018, as a substitute for Denzel Dumfries at half-time of a 1–4 friendly defeat against Belgium.