The main centres in New Tecumseth are the communities of Alliston, Tottenham, and Beeton. The Township of Tecumseth was reincorporated as the Town of New Tecumseth in 1991. The name 'New Tecumseth' was chosen because a Town of Tecumseh already exists in Essex County. The borders of the old township, with some adjustments along the eastern edge, and with the addition of all of Alliston, are the borders of the new town.
The town also includes the smaller communities of Allimil, Green Briar, Nicolston, Penville, Randall, Rich Hill, Schomberg Heights and Thompsonville.