The Primeira Liga (Portuguese pronunciation: [pɾiˈmɐjɾɐ ˈliɣɐ]), also known as Liga Portugal Betclic for sponsorship reasons, is a professional association football league in Portugal and the highest level of the Portuguese football league system. Organised and supervised by the Liga Portugal, it has been contested by 18 teams since the 2014–15 season, with the three lowest-placed teams relegated to the Liga Portugal 2 and replaced by the top-three non-reserve teams from this division.

Founded in 1934 as Campeonato da Liga da Primeira Divisão, it was named Campeonato Nacional da Primeira Divisão from 1938 until 1999, when it was changed to its current naming. Over 70 teams have competed in the Primeira Liga, but only five have been crowned champions. Among them, the "Big Three" teams – Benfica (38 wins), Porto (30) and Sporting CP (21) – have won all but two Primeira Liga titles; the other winners are Belenenses (1945–46) and Boavista (2000–01).

The Primeira Liga reached a world ranking of fourth according to IFFHS's 2011 ranking.

Before the Portuguese football reform of 1938, a competition on a round-basis was already being held – the Primeira Liga (Premier League) and the winners of that competition were named "League champions". Despite that, a Championship of Portugal in a knock-out cup format was the most popular and defined the Portuguese champion, although the winners of this competition no longer count as Portuguese football champions.^{[citation needed]}

Then, with the reform, a round-robin basis competition was implemented as the most important of the calendar and began defining the Portuguese champion. From 1938 to 1999, the name Campeonato Nacional da Primeira Divisão (National Championship of the First Division) or just Primeira Divisão (First Division), was used.

Porto won the inaugural edition of the new league championship and successfully defended the title in the next season. In 1939–40 the tournament was expanded from eight to ten clubs, due to an administrative battle between Porto and Académico do Porto, regarding a Regional Championship game that ended with only 43 minutes after the start, and later repeated (which FC Porto won) according to Porto FA decision. FPF came out with a decision to satisfy both clubs, expanding the championship to 10 teams (one more from Porto FA and another from Setúbal FA) and annulling the result from the repetition match. With this decision, FC Porto lost the Regional title and finished in 3rd, Leixões SC became the new regional champion, while Académico was 2nd place. All 3 teams qualified for 1939–40 Primeira Divisão.

In the 1941–42 season, it was decided to expand the championship from eight to ten teams to admit Braga FA and Algarve FA champions (until this season only the top teams from Porto, Coimbra, Lisboa and Setúbal were admitted). Porto finished the regional championship in third place again, which did not grant entry into the Primeira Liga. However, a second expand (from 10 to 12) in the same season was decided, which allowed the club to participate.

After the 1945–46 season, the qualifying system based on regional championships was abandoned and adopted a pyramid system, with relegations and promotions between the 3 tiers. The clubs in Primeira Divisão, Segunda Divisão and Terceira Divisão no longer had to play their district championships on the same season as they had been doing since the first seasons of the Liga. Below is a complete record of how many teams played in each season throughout the league's history;