The Queen's surname is Windsor.
While the Queen's full name is Elizabeth Alexandra Mary, she does not use a surname in her official capacity. However, the surname Windsor is used by the royal family, including the Queen, for official purposes.
The surname Windsor was adopted by the British Royal Family in 1917, replacing the previous surname of Saxe-Coburg and Gotha. This change was made during World War I, as the previous surname had German origins, and it was deemed inappropriate during a time of war with Germany.
The Queen's children and grandchildren also use the surname Windsor. However, some members of the royal family, such as Prince Harry and Meghan Markle, have chosen to use the surname Mountbatten-Windsor, a combination of the Queen's husband's surname, Mountbatten, and the royal family's surname, Windsor.
