Alan Leo, born William Frederick Allan, (Westminster, 7 August 1860 - Bude, 30 August 1917), was a prominent British astrologer, author, publisher and theosophist, and is considered by many to be the father of modern astrology.

Leo, who took the name of his sun-sign as a pseudonym, founded the Astrological Lodge of the Theosophical Society in 1915.

He is credited as being one of the most important astrologers in the 20th century because it appears that his work had the effect of stimulating a revival of astrology in the west after its general downfall in the 17th century. Leo was a devout Theosophist and he worked many of religious concepts such as karma and reincarnation into his astrology. He used the Theosophical Society’s vast international connections to publish, translate and disseminate his work across Europe and America and it was in these countries that astrology began to be revived.