Therese Elisabeth Alexandra Förster-Nietzsche (10 July 1846 – 8 November 1935) was the sister of philosopher Friedrich Nietzsche and the creator of the Nietzsche Archive in 1894.

Förster-Nietzsche was two years younger than her brother. Their father was a Lutheran pastor in the German village of Röcken bei Lützen. The two children were close during their childhood and early adult years. However, they grew apart in 1885, when Förster-Nietzsche married Bernhard Förster, a former high school teacher who had become a prominent German nationalist and antisemite. Nietzsche did not attend their wedding.

Förster-Nietzsche and her husband created an unsuccessful colony, Nueva Germania, in Paraguay in 1887. Her husband committed suicide in 1889. Förster-Nietzsche continued to run the colony until she returned to Germany in 1893 where she found her brother to be an invalid whose published writings were beginning to be read and discussed throughout Europe. Adolf Hitler attended her funeral in 1935.

In the 1950s, it was claimed by Nietzsche's new editors and translators such as Walter Kaufmann that Nietzsche's work had been edited by Förster-Nietzsche to highlight racist and eugenicist themes, but this account has been the subject of debate in recent scholarship. An alternative theory exonerates Förster-Nietzsche and places the distortion of Nietzsche's works in the hands of the Nazis themselves.

Therese Elisabeth Alexandra Nietzsche was born in 1846 to Carl Ludwig Nietzsche and Franziska Nietzsche (née Oehler). She was named after three princesses with whom Carl Ludwig Nietzsche had worked. Carl Ludwig was a Lutheran pastor in the German village of Röcken bei Lützen. Franziska was a rustic. Carl Ludwig died in 1849. Franziska had no prospects and her husband's pension was insufficient. She chose to rely on the charity of Carl Ludwig's mother, Erdmuthe, and the more distinguished prospects which she could open for the children. When remembering her early life, Förster-Nietzsche would suggest that they may have cried a lot.

Friedrich and Förster-Nietzsche were close during their childhood and early adult years. In 1864, she suggested "that she join him in the new manner of free-thinking he had adopted." He replied that, if she wanted to follow him "into higher intellectual realms" she would have to choose between "views about the world, God, and atonement held by family and society, and the difficult, convention-defying search for truth." Nietzsche took to calling her "Llama" throughout their lives because he felt that the description of the load bearing, saliva spitting, stubborn animal fit her well.

Bernhard Förster planned to create a settlement in the New World and had found a site in Paraguay which he thought would be suitable. The couple persuaded fourteen German families to join them in the colony, to be called Nueva Germania, and the group left Germany for South America on 15 February 1887.

The colony did not thrive. The German methods of farming were not suitable to the land, illness ran rampant, and transportation to the colony was slow and difficult. Faced with mounting debts, Förster committed suicide by poisoning on 3 June 1889. Four years later, Förster-Nietzsche left the colony forever and returned to Germany. The colony still exists as a district of the San Pedro department.