Ernst II Bernhard Georg Johann Karl Frederick Peter Albert, Duke of Saxe-Altenburg (b. Altenburg, 31 August 1871 – d. Trockenborn-Wolfersdorf, 22 March 1955), was the last reigning duke of Saxe-Altenburg.

He was the fourth child but only son of Prince Moritz, the youngest son of Georg, Duke of Saxe-Altenburg and Princess Augusta of Saxe-Meiningen.

The death of his father on the 13 May 1907 made him first in the line of succession to the duchy of Saxe-Altenburg. He inherited the dukedom when his uncle and namesake Ernst I, died without surviving male issue, on the 7 February 1908.

On 27 February 1898 in Bückeburg Ernst married his first wife, Princess Adelaide of Schaumburg-Lippe, a granddaughter of George William, Prince of Schaumburg-Lippe. They had four children:

During World War I, Ernst refused all honorary appointments at the Kaiser's headquarters (which would have been considerably safer than other areas). He renounced his rank as General, entered the service as a mere Colonel, and distinguished himself at the Battle of Péronne. By the end of the war, he was a commander of a division.

A great lover of science, Ernst had a wireless installation fitted inside his castle in Altenburg during the start of the war. Its purpose was to specially communicate with airships. Ernst also had a lifelong interest in wireless telegraphy and telephony, and was considered an expert of aeronautics.

When Germany lost the war, all the German princes lost their titles and states. Ernst was one of the first princes to realize major changes were coming for Germany, and quickly arrived at an amicable settlement with his subjects. He was forced to abdicate the government of the duchy on 13 November 1918, and spent the rest of his life like a private citizen.

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