Elsa Einstein (18 January 1876 – 20 December 1936) was the second wife and first cousin of Albert Einstein. Their mothers were sisters, and further, their fathers were first cousins. Elsa had the surname of Einstein at birth, lost it when she took the name of her first husband Max Löwenthal, and regained it in 1919 when she married her cousin Albert.
Elsa, the daughter of Rudolf Einstein, was born in Hechingen in January 1876. She had two sisters; Paula (c. 1878–c. 1955) and Hermine (1872–1942). Rudolf was a textile manufacturer in Hechingen. During the regular visits with the family in Munich, she often played with her cousin Albert. In her Swabian dialect, she called him "Albertle". The two parted ways in 1894, when Albert left Germany to follow his family to Milan.
In 1896, Elsa married textile trader Max Löwenthal (1864–1914), from Berlin, with whom she had three children: daughters Ilse (1897–1934) and Margot (1899-1986), and a son who was born in 1903, but died shortly after birth. They lived together in Hechingen. In 1902, Max Löwenthal took a job in Berlin. His family stayed in Hechingen. She divorced Max on 11 May 1908, and moved with her two daughters to an apartment above her parents on Haberlandstrasse 5, in Berlin.
She began a relationship with her cousin Albert Einstein in April 1912, while Albert was still married to his first wife, the physicist Mileva Marić. Einstein separated from Mileva in 1914 and their divorce was final on 14 February 1919. Elsa married him three and a half months later, on 2 June 1919.
Elsa's and Albert's mothers were sisters, which made Elsa and Albert maternal first cousins, and their fathers were first cousins. Ilse and Margot, Albert Einstein's first cousins once removed, had already changed their surname to Einstein and were now also his stepdaughters.
With daughters Ilse and Margot, the Einsteins formed a close-knit family. Although Albert and Elsa did not have any children of their own, Albert raised Ilse and Margot as his own. They lived in the Berlin area, also having a summer house in Caputh in nearby Potsdam.
Elsa spent most of her marriage with Albert acting as gatekeeper, protecting him from unwelcome visitors and charlatans. She also was the driving force behind building their summer house in 1929.
In 1933, Albert and Elsa Einstein emigrated to Princeton, New Jersey, US. In autumn 1935, they moved to a house at 112 Mercer Street, bought that August, but shortly afterwards Elsa developed a swollen eye and was diagnosed with heart and kidney problems. When Elsa was diagnosed, Einstein decided to spend much of his time in his studies. It was stated in Walter Isaacson's book, Einstein: His Life and Universe, that he believed "strenuous intellectual work and looking at God's nature are the reconciling, fortifying yet relentlessly strict angels that shall lead me through all of life's troubles". Thus did Einstein try to escape from his troubles by focusing on work that would distract him from Elsa's dying. Elsa wrote that Einstein would "wander around like a lost soul". When Elsa died, Einstein was heartbroken. It was even said by his friend Peter Bucky that Einstein cried, which Peter had never seen before. Elsa died after a painful illness on December 20, 1936, in the house on Mercer Street.