Eric II, Duke of Brunswick-Lüneburg (10 August 1528 – 17 November 1584) was Duke of Brunswick-Lüneburg and ruler of the Principality of Calenberg from 1545 to 1584. Since 1495 the Principality of Göttingen was incorporated in Calenberg.
He was the son of Eric I and Elisabeth of Brandenburg. While he was still a minor, his mother acted as Regent and introduced the Reformation in Calenberg, including her children's conversion. However, in 1547 Eric II declared his reconversion to Roman Catholicism, to the dislike of his mother.
Eric married on 17 May 1545 Sidonie of Saxony (1518–1575), who was ten years his senior. The wedding ceremony was held in Hann. Münden without the usual pomp and circumstance.
Initially, they liked each other. Eric had been engaged to Agnes of Hesse. When the marriage was negotiated at the court in Kassel, however, he had met Sidonie. He liked her, and broke off the engagement with Agnes, in order to marry Sidonie. Landgrave Philip I of Hesse predicted: "All sorts of things will happen inside this marriage after the kissing month ends."
Two years into the marriage, in 1547, Duke Eric began his rule and reconverted to the Catholic faith, after the Reformation had been introduced to his Duchy in 1542. Despite her husband's pleas, Sidonie held on to her Lutheran faith. They had financial problems and the marriage remained childless marriage, and soon their relationship took a very unfortunate course.
The clashes culminated in her suspicion that her husband wanted to poison her. A Genoese merchant had contacted Sidonie's brother Augustus in 1555 and informed him that Duke Eric I had ordered poison from him on the grounds that "Eric was a Christian and his wife would be Lutheran, it was better that one woman part was destroyed, than 20,000 people."