Clubs competing in the S.League are also required to participate in the S.League Reserve cum Youth Tournament, also known as the  Prime League. Prime League sides also take part in the National Football League FA Cup competition.

All Prime League teams shall register a minimum of 15 and a maximum of 20 players. Any teams not participating in the S.League shall register a minimum of 20 and a maximum of 25 players. The teams must maintain the minimum required number of players throughout the season.

Each club is allowed to register up to a maximum of only ONE foreign player for their Prime League teams. The foreign players shall be aged eighteen and above at the point of registration but under twenty one years of age and is eligible to play in the S.League provided he replaces one of the club's S.League foreign players, and the player being replaced must not be under suspension or be ineligible for any reason.

An S.League player who has played in his team’s previous S.League match is not eligible to play in the following Prime League match or matches. Players who did not play in their team’s previous or last S.League match, as aforesaid, are not included in this category.