The Diary of Friedrich Kellner is a journal written during the Second World War. The author, August Friedrich Kellner, was a justice inspector in the courthouse in Mainz. He was also a political activist for the Social Democratic Party of Germany between 1918 and 1933. After Hitler banned the party, the family moved to Laubach.

When Hitler ordered the invasion of Poland in September 1939, Kellner began his secret diary. He named his diary "Mein Widerstand," which means "My Opposition." In the diary, he wrote down what he saw and heard about what happened in Nazi Germany. He wrote it so that people later would know about it.

The diary has ten volumes, with a total of 861 pages. There are 676 dated entries. Included in the diary are more than 500 newspaper clippings.

The English translation of the diary was published in 2018 by Cambridge University Press under the title, My Opposition: The Diary of Friedrich Kellner -- A German against the Third Reich.