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Constitution in English and Portuguese (7th revision, 2005), accessed May 30, 2024: 

Article 121: 1. The President of the Republic is elected by the universal, direct and secret suffrage of Portuguese citizens who are registered to vote in Portuguese territory and, in accordance with the following paragraph, of Portuguese citizens who reside abroad. 

2. The law shall regulate the right to vote of Portuguese citizens who reside abroad, to which end it must pay due regard to the existence of ties that effectively link them to the Portuguese community. Article 115:12. Citizens who reside abroad and are properly registered to vote under the provisions of Article 121(2) shall be called upon to take part in referenda that address matters which specifically also concern them.

Law governing Elections to the Assembly of the Republic Portuguese and English (14/79 of 16 May 1979, last amended on 11 November 2020), accessed 30 May 2024: 

Article 79-E Casting a vote in advance by electors who are abroad 

1 - Electors who are in any of the situations provided for in Article 79-B(2) may vote between the twelfth and the tenth days before that of the election, at the diplomatic or consular missions or the foreign delegations of the Portuguese ministries and public institutions indicated beforehand by the Ministry of Foreign Affairs, in accordance with Article 79-C(7) to (14). 

2 - The functions provided for in Article 79-C (8) to (13) shall be carried out by a diplomatic officer appointed for this purpose, who shall send the electoral correspondence by the most expeditious means to the respective parish council. 

3 - In the case of voters referred to in Article 79-B(2)(a), if the Ministry of Foreign Affairs recognizes that it is impossible for them to travel to the places referred to in paragraph 1, it will appoint a diplomatic official to collect the electoral correspondence in the period specified therein. 

4 - The electoral operations provided for in the preceding paragraphs can be inspected by the lists who may appoint delegates until the sixteenth day before that of the election.

Legal Regime governing Referenda in Portuguese and English (Law no. 15-A/98 of 3 April 1998, last amended by Organic Law no. 4/2020 of 11 November 2020), accessed 30 May, 2024: 

Article 37:2 When a referendum addresses a matter that also specifically concerns them, citizens who reside abroad and are properly registered on the electoral roll under the provisions of Article 121(2) of the Constitution shall also be called upon to participate. 

Article 128:2 When the electors referred to in subparagraphs a), b) and g) of the previous paragraph are away abroad between the twelfth day before that of the referendum and that on which the referendum is held, they may exercise the right to vote at the diplomatic or consular representations or the foreign delegations of the Portuguese ministries and public institutions defined in advance by the Ministry of Foreign Affairs, in accordance with Article 130-A. 

Legislative Initiative by Citizens in Portuguese and English (Law no. 17/2003 of 4 June 2003, last amended by Law no. 51/2020 of 25 August 2020), accessed 30 May 2024: 

Article 2: Citizens who are definitively registered on the electoral roll in Portuguese territory or abroad are holders of the right to initiate legislation.

For an extended review of external voting developments in Portugal, refer to ​Voting from Abroad: The International IDEA Handbook 2007 (pages 83-87):

“More recently, 1997 marked a major change in the process of external voting in Portugal. The constitution was revised for the fourth time, and one of the specific topics for reform was the rules governing elections. Only minor changes were introduced, but in this constitutional package, agreed by the two major parties, the centre–left Socialist Party (Partido Socialista, PS) and the centre–right Social Democratic Party (Partido Social Demócrata, PSD), Portuguese citizens resident abroad became eligible to vote in both presidential elections and national referendums as well as legislative elections. This had been promoted in particular by the PSD in the constitutional negotiations. (The experience of legislative elections since 1976 had shown that the PSD has consistently had a majority of votes among external voters, which helps to explain why that party lobbied for an extension of emigrants’ voting rights.) These changes equalized the electoral rights of Portuguese citizens inside and outside Portugal for all national-level elections, although external voting was not extended to local elections. Portuguese citizens with external voting rights were first able to exercise their right to vote for the president of the republic in 2001.” [...] 

Decree-Law no. 95-C/76 of 30 January 1976 sets out the procedural aspects of external voting in legislative elections. In these elections, as well as in European Parliament elections, the external voting right is exercised by post.  [...] 

For elections to the national parliament and the European Parliament (from 1987 to 2004), the Ministry of Internal Administration in Portugal sends a ballot paper with two envelopes to all citizens registered in electoral registers abroad around two to three weeks before polling day. The voter fills in his or her ballot paper, folds it and inserts it in one of the envelopes which he or she then seals and places in the other envelope together with a photocopy of his other elector’s card. All must be sent back to the designated reception point in Portugal and can be posted up until (and including) the day of the election. Ten days following the election, the external ballot papers are opened and counted. In legislative elections, the four parliamentary seats reserved for external voters are then allocated. In European Parliament elections, the external votes are added to the internal national totals (Decree-Law no.95C/76). In presidential elections and referendums, Portuguese residents abroad have to vote in person. In these cases, external voting takes place over a period of three days up to and including election day. Registered electors can go to the voting centre closest to their place of residence to cast their vote, which is then opened and counted. [...] 

The constitutional revision of 1997 introduced a number of modifications to the referendum law. It allows Portuguese nationals resident abroad to vote in referendums, provided they are correctly registered, whenever the referendum deals with issues that concern them specifically. This last provision is ambiguous and lends itself to significant political argument about whether in any given referendum Portuguese citizens resident abroad may or may not vote. In practice, there have been two referendums in Portugal: one in 1998 on abortion and another in 1999 on the issue of regionalization. In both instances, following Constitutional Court rulings (Ruling 288/98 and Ruling 532/98), external voting was not allowed."