Named for its principal river. This is one of the most ancient names on the map of Wisconsin, and was so designated in honor of an early French voyageur who was wrecked at its mouth -- see Wis. Hist. Colls., xvi, p. 185.  It may also have been named "St. Croix" by French missionaries because the river by that name enters the Mississippi at a point resembling the "holy cross".

Description from John W. Hunt's 1853 Wisconsin Gazetteer: "ST. CROIX, County, is bounded on the north by La Pointe, on the east and south by Chippewa, on the southwest and west by the boundary between the State and Minnesota. The county seat is at Hudson, formerly Willow river, at the mouth of a stream of the same name, emptying into Lake St. Croix. It was set off from Crawford, and organized January 29, 1850. The boundaries were somewhat changed March 16, 1849.  It is one of the largest counties in the State, being 130 miles in length, and 50 in width; presents to the agriculturist, in fertility of soil, well watered and well wooded farms, in the means of access to market through Lake St. Croix and the Mississippi, and in the perfect healthiness and salubrity of climate, advantages which are to be found combined in but few places in the West. The surface is generally undulating north of the Falls of St. Croix. It is mostly timbered with maple and other hard woods, while south of the Falls is a due proportion of prairie and openings. But little attention has yet been paid to the pursuits of agriculture, and the manufactories are confined for the present to pine lumber. It is well watered with fine streams and beautiful lakes. The principal streams are Willow, Kinnickinnic, Vermillion, Isabelle, and Rush river. Population in 1846 was 1,419; in 1847, 1,674; in 1850, 624; with 181 dwellings, 4 farms, and 2 manufactories. In 1846 the census returns included all of the present Territory of Minnesota, east of the Wisconsin river, also the present county of La Pointe. In 1847 it included the same, excepting the county of La Pointe. This is the reason why there appears to be a decrease in the population from 1847 to 1850."

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[Source: Kellogg, Louise Phelps. "Derivation of County Names" in Proceedings of the State Historical Society of Wisconsin for 1909, pages 219-231; Milwaukee Sentinel, Jul. 16, 1948.]