Father Jacques Marquette, S.J. (1636 - May 19, 1675) was a Jesuit missionary and the first European to discover the Mississippi River.

Father Marquette was born in the village of Laon in France, and joined the Society of Jesus at age seventeen. After working and teaching in France for several years, he was dispatched to Quebec in 1666 to missionize to the Native Americans, where he showed great proficiency in the local languages, especially Huron.

In 1668 Father Marquette was redeployed by his superiors to missions farther up the St. Lawrence River in the western Great Lakes. He worked at Sault Ste. Marie and at the Mission of the Holy Spirit in La Pointe, on Lake Superior, near the present-day town of Ashland. Here, he came into contact with members of the Illinois tribes, who told him of the existence of the Mississippi River and invited him to come teach further south. Due to wars between the Hurons at La Pointe and the neighboring Dakota people, however, Father Marquette had to relocate to the Mackinac Straits, where he informed his superiors about the rumored river, and requested permission to explore it.

Leave was granted, and in 1673 Marquette was joined by Louis Joliet, a French Canadian explorer. They departed from Mackinac on May 17, with two canoes and five other Frenchmen. They followed Lake Michigan to Green Bay and up the Fox River. From here, they portaged to the Wisconsin River, which they were told led to the river they sought. On June 17 they entered the Mississippi, becoming the first Europeans to do so.

The Joliet-Marquette expedition travelled to within 700 km of the Gulf of Mexico, but turned back at the mouth of the Arkansas River. By this point they had encountered a number of natives carrying European trinkets, and they feared any encounter with explorers or colonists from Spain. They followed the Mississippi back to the mouth of the Illinois River, which they learned from local natives was a shorter route back to the Great Lakes. They returned to Lake Michigan at the point of modern-day Chicago, Illinois. Marquette stopped at the mission of St. Francis Xavier in Green Bay in September, while Joliet returned to Quebec to relate the good news of their discoveries.

Marquette returned to the Illinois River in 1674 to found a mission among the Illinois people on the site of Chicago on December 4, 1674 (the mission would later grow into the city of Chicago). A bout of dysentery picked up during the Mississippi expedition, however, had sapped his health. On the return trip to Mackinack he died near the modern town of Ludington, Michigan.

Father Marquette is memorialized in several towns (such as Marquette, Michigan) and rivers that bear his name, as well as the Father Marquette National Memorial near St. Ignace, Michigan. He is also the namesake of Marquette University in Milwaukee.