Sir William Grenfell Max Muller (born Wilhelm; 9 June 1867 – 10 May 1945) was a British diplomat.

Appointed a CB in 1911, he served at the Foreign Office from 1911 to 1912, and was a British delegate to the International Opium Conference at The Hague in 1911 and 1913.

In 1913, he was sent to Budapest as consul-general: although a consular post, it was always held by a diplomat, owing to the duties involved.

On the outbreak of the First World War in 1914, he returned to London in August and was employed in the Foreign Office.

Lady Max-Muller was painted by Philip de László, but later accused him of disloyalty during the First World War.