Catherine I (Russian: Екатерина I Алексеевна; 15 April 1684 – 17 May 1727) was the second wife of Peter the Great and Empress of Russia from 1725 until her death. There are no documents that confirm her origins. Said to have been born on 15 April 1684, she was originally named Marta Helena Skowrońska. Marta was the daughter of Samuel Skowroński, a Roman Catholic peasant from the Polish–Lithuanian Commonwealth born to Minsker parents, who in 1680 married Dorothea Hahn at Jakobstadt. Some biographies state that Marta's father was a gravedigger and handyman, while others speculate that he was a runaway landless serf. In 1704, she was well established in the Tsar's household as his mistress, and gave birth to a son, Peter. Though no record exists, Catherine and Peter are described as having married secretly between 23 Oct and 1 Dec 1707 in St. Petersburg. They had twelve children, two of whom survived into adulthood, Anna (born 1708) and Yelizaveta (born 1709). Catherine went with Peter on his Pruth Campaign in 1711. There Catherine was said to have saved Peter and his Empire. In any case Peter credited Catherine and proceeded to marry her again (this time officially) at Saint Isaac's Cathedral in St. Petersburg on 9 February 1712. Catherine was Peter's second wife; he had previously married and divorced Eudoxia Lopukhina, who had borne him the Tsarevich Alexis Petrovich. Upon their wedding, Catherine took the style of her husband and became Tsaritsa. In 1724 Catherine was officially crowned and named co-ruler. Peter died (28 January 1725 Old Style) without naming a successor. Catherine represented the interests of the "new men", commoners who had been brought to positions of great power by Peter based on competence. For that reason during a meeting of a council to decide on a successor, a coup was arranged by Menshikov and others in which the guards regiments with whom Catherine was very popular proclaimed her the ruler of Russia, giving her the title of Empress. The real power, however, lay with Menshikov, Peter Tolstoy and with other members of the Supreme Privy Council.

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