The Russian monarch known as "the Great Reformer" is Tsar Peter I, more commonly known as Peter the Great. He ruled Russia from 1682 to 1725 and is credited with modernizing and westernizing the country. Peter implemented a series of reforms in government, military, and culture, aimed at transforming Russia into a major European power. He reorganized the military, established a navy, and introduced new administrative systems. His ambitious projects included the founding of St. Petersburg, which became Russia’s new capital, symbolizing his vision for a modern and progressive state.