| Álvaro of Lencastre | |
| Succession: | 3rd Duke of Aveiro |
| Reign: | 1598 - 1626 |
| Predecessor: | George, 2nd Duke of Aveiro |
| Successor: | Raimundo, 4th Duke of Aveiro |
| Spouse: | Juliana of Lencastre |
| Issue: | George, 1st Duke of Torres Novas Afonso of Lencastre Madalena Maria Violante Peter, 5th Duke of Aveiro Louís |
| House: | House of Aviz |
| Father: | Afonso of Lencastre |
| Birth Date: | 1540 |
| Death Date: | 1626 |
Álvaro of Lencastre (1540–1626) was the son of Afonso of Lencastre, second son of infante George of Lencastre, 2nd Duke of Coimbra.
When George of Lencastre, 2nd Duke of Aveiro died, in 1578 in the Battle of Alcácer Quibir, together with King Sebastian I of Portugal and most Portuguese nobles, the Dukedom of Aveiro was claimed by two pretenders:
According to the Portuguese: Lei Mental ("Mental Law"), females could not inherit their father’s lands and fiefs (except with a specific royal permission). That was why Álvaro of Lencastre claimed his cousin's inheritance, to prevent the extinction of such a remarkable aristocratic House.
This dispute took about 2 decades, and finally, King Philip I of Portugal (also known as Philip II of Spain) decided that Dom Álvaro should marry his cousin, Dona Juliana, in order to inherit his family titles and estates, and they became jointly 3rd Dukes of Aveiro.
This marriage took place in 1598 and, for that occasion, the King granted the Dukes special honours:
The Ducal Palace was located in Azeitão, where, attached to the palace, they also built a hospital. They granted a huge protection to the Arrábida convent, founded by the 1st Duke, John of Lencastre, where they both are buried.
The couple had sixteen children: