Son of Sigismund the Elder, King of Poland (1548-1572) and Grand Duke of Lithuania (1544-1572), during whose reign Poland and the Grand Duchy of Lithuania united to form a confederation - the Republic of the Two Nations. He was the last ruler of the Jogailacian dynasty.
Jigimon Augustus was the son of Sigismund the Old and his second wife, Bonn Sforza. He made sure that his descendant would inherit the throne after his death. As a result, in 1522, Sigismund Augustus was proclaimed heir to the throne by the Seimas of the Grand Duchy of Lithuania. In 1526, Lithuanian nobles proposed to the King to change the form of government of the Grand Duchy of Lithuania to that of a kingdom, and to proclaim Sigismund Augustus as King, but they were not supported.
In 1529, after his mother's intrigues swayed the nobility to her side, and after Sigismund the Old had agreed to approve the Statute of Lithuania I, which further expanded the rights of the nobility, Sigismund Augustus was proclaimed Grand Duke of Lithuania at the age of just 8. His inauguration as Grand Duke of Lithuania in Vilnius in 1529 was the last ceremony in which subsequent rulers of the Republic of the Two Nations were not inaugurated as Grand Dukes of Lithuania separately.
At the Diet of Piotrkow in 1529, he was also elected King of Poland to ensure that the dynastic link between the Grand Duchy of Lithuania and the Kingdom of Poland was not broken. He was elected King of Poland on the condition that he would seek the accession of Lithuania and Prussia to the Polish Crown. Sigismund Augustus did not become the actual ruler of Poland until after his father's death in 1548.
Sigismund Augustus began to rule Lithuania on his own in 1544, when a party led by Albert Goštautas was formed [in Lithuania], demanding a greater separation from the Polish Kingdom. The members of this opposition complained that the Grand Duke of Lithuania lived too little in Lithuania. This was brought to the attention of Sigismund the Old through a special delegation sent by the Seimas of the Grand Duchy of Lithuania (1538). As a result, in 1543 Sigismund the Elder had the idea of entrusting his successor, who was already 23 years old, to rule Lithuania. The Lithuanian nobles were comfortable with this idea. They wanted to have a lord's court in Vilnius and to protect themselves from the interference of Polish nobles in Lithuanian affairs. The Grand Duke's residence in Kraków would allow the Poles to interfere in Lithuanian affairs by influencing him.
In 1544, Sigismund Augustus settled in Vilnius as Grand Duke of Lithuania, and he was given full power in Lithuania. Sigismund the Old retained only the treasury and foreign affairs. After his father's death in 1548, Sigismund Augustus became the ruler of the Republic of the Two Nations without an electoral procedure.
