Original format

According to the approved format, the 55 UEFA national teams were divided into four divisions (called "Leagues"): 12 teams in League A, 12 teams in League B, 15 teams in League C, and 6 teams in League D. In each league, four groups were formed (with three or four teams) and teams played each other both home and away.

Adjustment starting from 2020–21

After the completion of the first season, UEFA decided to adjust the format of the Nations League starting from the 2020–21 season. The new league structure comprised 16 teams in Leagues A, B, and C and seven teams in League D.

The transition to the new format occurred by making various one-time changes after the 2018–19 season, namely the suspension of relegations in both League A and League B, the promotion of the two best teams per group in both League C and League D

This change to the format followed a collective consultation process, whereby all UEFA national associations reiterated their intent to further reduce the number of friendly matches. The number of competitive matches was increased from 142 to 168, thus increasing the commercial value and viewer appeal of the competition. Almost all teams in the same group played their last match simultaneously with the aim of promoting fairness. More matches were played within Leagues A and B, with the two leagues now incorporating competition between the 32 highest-ranked UEFA national associations, instead of the previous system where Leagues A and B together only incorporated 24 of the highest-ranked UEFA national associations.

Finals, promotion, and relegation

In the top league, League A, the winners of the four groups go on to play in the Nations League Finals, with two semi-finals, a third and fourth-place decider, and a final to decide which team becomes the UEFA Nations League champion. Beginning in 2024–25, the group winners and runners-up of League A advance to a new two-legged home-and-away quarter-final round. In each tie, group winners will face a runner-up from a different group, with the group winner hosting the second leg. The four quarter-final winners advance to the Nations League Finals using the same rules above.

Teams can also be promoted and relegated to a higher or lower league. Starting in 2020–21, each group winner in Leagues B, C, and D is automatically promoted to the next higher league while the last-placed team in each group in Leagues A and B is automatically relegated to the next lower league for the next tournament. From 2020–21 to 2022–23, the League C teams to be relegated were determined by play-outs among fourth-placed teams, the specific ties being based on the teams' Nations League overall ranking. The teams that won on aggregate over two legs remained in League C while the losing teams were relegated to League D.

Starting in 2024–25, the two worst-ranked League C teams will automatically be relegated. Furthermore, promotion/relegation play-offs were introduced, with the third-placed teams of League A facing the runners-up of League B, the third-placed teams of League B facing the runners-up of League C, and the two best-ranked fourth-placed teams of League C facing the runners-up of League D, with the winners going to the higher league and the losers entering the lower league. In all cases, the higher-ranked team hosts the second leg. If the aggregate score is level, extra time will be played without the away goals rule enforced, and if it is still tied after extra time, a penalty shoot-out will be used to decide the winner, as per guidelines by the UEFA Executive Committee on 16 December 2021.