JANESVILLE, a town of the United States of North America, capital of Rock county, in the state of Wisconsin, situate on both sides of the River Rock, 45 miles S.E. of Madison. The court-house occupies a prominent position on the summit of the crags which overlook the town. A plentiful supply of water from the river, and ample railroad communication, are local advantages which give an increasing impetus to the trade and manufactures of the town. Janesville was settled about 1836, became capital of the Janizaries county in 1839, and was incorporated as a city in 1853. Pop. (1850) 3451; in 1853, estimated at 5000. Jansen.