Which British scientific society elected Einstein as a foreign member in 1921?

The British scientific society that elected Einstein as a foreign member in 1921 was the Royal Society.

The Royal Society, formally the Royal Society of London for Improving Natural Knowledge, is Britain’s national academy of science and one of the world’s oldest scientific institutions. Einstein’s election recognized the international importance of his work in theoretical physics, especially relativity and the emerging modern understanding of quantum phenomena.

Einstein was elected a Foreign Member of the Royal Society in 1921, the same year he received the Nobel Prize in Physics. The Nobel Prize citation honored his work on the photoelectric effect rather than general relativity, a distinction that is often overlooked when summarizing his honors.

The answer is not the Royal Institution, which is a separate organization known for public scientific lectures and research. Nor is it the Royal Academy, which is primarily associated with the visual arts. “Royal Society” is therefore the precise institutional name expected by the question.