The 1st Legislature of the Third Portuguese Republic () ran from 3 June 1976 to 12 November 1980. The composition of the Assembly of the Republic, the legislative body of Portugal, was determined by the results of the 1976 legislative election, held on 25 April 1976.
This was the only legislature where the composition changed mid-term due to another election, when the 1979 legislative election, held on 2 December 1979, was called after a period of governmental instability, with the resignation of Mário Soares government in 1978, plus the fall of two Presidential appointed Prime Ministers also in 1978 and 1979. President António Ramalho Eanes dissolved the Assembly in September 1979 and called an election for 2 December 1979. Due to the decree that approved the Constitution, which stipulated that the first legislature had a fixed date of completion on 14 October 1980, another election had to be held at end of the of the legislature in 1980.
The 1st Portuguese legislative election was held on 25 April 1976. In the election, the Socialist Party (PS) won the most seats and formed a minority government. Between January and August 1978, the PS formed a majority coalition government with the Democratic and Social Center (CDS).
The 2nd Portuguese legislative election was held on 2 December 1979. In the election, the Democratic Alliance (AD), a coalition composed by the Social Democratic Party (PPD/PSD), the Democratic and Social Center (CDS) and the People's Monarchist Party (PPM) won a majority of the seats and the three parties formed a majority coalition government.
To be elected, a candidate needs to reach a minimum of 132 votes. Vasco da Gama Fernandes, from the Socialist Party, was easily elected:
At the end of each parliamentary session, the regiment called for a new election to elect the President. As such, a new ballot was held in the beginning of the second session, where Vasco da Gama Fernandes was re-elected.
At the beginning of the third session, Vasco da Gama Fernandesàdecided not to run again for President. As such, the Socialist Party appointed Teófilo Carvalho dos Santosàas the new candidate. He was elected.
After the 1979 elections, the Social Democratic Party proposed Leonardo Ribeiro de Almeida as a candidate, who ran against the incumbent President, Teófilo Carvalho dos Santos. Leonardo Ribeiro de Almeida was elected.