Thac Niagara (tieng Anh: Niagara Falls; tieng Phap: les Chutes du Niagara) o song Niagara tai Bac My, nam o đuong bien gioi cua Hoa Ky va Canada. Thac Niagara bao gom 3 thac rieng biet: thac Horseshoe (Canada) (đoi luc goi la thac Canada), thac My va mot thac nho hon gan đo la thac Bridal Veil. Du thac khong cao nhung cac thac Niagara rat rong. Voi hon 168.000 m³ nuoc roi xuong moi phut vao thoi điem nhieu nhat, va trung binh gan 110.000 m³ moi phut, đay la thac nuoc manh nhat o Bac My. Thac Niagara toa lac khoang 20 phut đi tu thanh pho Buffalo cua Hoa Ky va Toronto cua Canada.

Cac thac Niagra noi tieng vi ve đep va nguon gia tri cho thuy đien va mot du an gay thach thuc cho bao ve moi truong. La mot đia điem du lich noi tieng mot the ky, ky quan thien nhien nay nam giua hai thanh pho ket nghia Niagara Falls, Ontario va Niagara Falls, New York.
Thac Niagara gom Thac Mong Ngua (Horseshoe Falls-thuoc Canada) va Thac My. Thac Horseshoe Falls cao khoang 173 feet (53m). Chieu cao thac My thay đoi tu 70–100 feet (21m) do khoi đa lon nam duoi chan thac. Thac Horseshoe Falls rong khoang 2600 feet (792 m), con Thac My American Falls rong 1.060 feet (323 m). 
The tich nuoc cua thac vao mua nuoc lon co the đat toi 202.000 cubic feet moi giay (5.720 m³/s). Suot mua he, moi giay khoang 2.832 m³ nuoc chay qua thac, trong đo toi 90% la qua thac Horseshoe Falls.


Vuot thac Thang 10 nam 1858, Sam Patch –nhay thac va song sot, mo ra 'truyen thong' daredevils vuot thac. Vao ngay 24 thang 10 nam 1901, co giao 63 tuoi o bang Michigan ten la Annie Edson Taylor đa la nguoi đau tien vuot thac trong mot cai thung; Ba nay cung song sot, tuy bi xay xuoc mot chut nhung hau nhu binh yen vo su.
Sau ba Taylor, 14 nguoi khac cung muon vuot thac trong hoac tren mot phuong tien nao đo. Mot vai nguoi khong he han gi nhung đa so bi thuong nang. Nhung nguoi song sot sau nhung tro pho truong nay đeu bi buoc toi va bi phat boi vi luat cam nhay thac đa đuoc đua ra o ca hai nuoc.
Đieu ky dieu o Niagara "Miracle at Niagara" xay ra ngay 9 thang 7 nam 1960, mot cau be 5 tuoi nguoi My ten la Roger Woodward đa song sot khi roi tu tren đinh thac Horseshoe Fall xuong ma chi đuoc bao ve boi mot chiec ao phao. Co chi gai cua cau, Deanne, đuoc cuu khi chi con cach mieng thac chung 20 feet (6m). Vai phut sau, Roger đuoc cuu o phia duoi chan thac boi nhung nguoi tren tau Maid of the Mist (Tau du lich o thac Niagara). Su song sot kho tin cua cau đa đuoc đua tin khap the gioi.


Xem them Thac Iguazu


Tham khao Lien ket ngoai Panorama Niagara Falls Panorama found at Queen's Park, Toronto.
Historic Niagara Digital Collections
U.S. Army Corps of Engineers The U.S. Army Corps of Engineers completely blocked the flow of water over the American Falls in 1969.
The History of Niagara Falls
"Niagara Power Goes Under Ground" Popular Mechanics, April 1952, pp. 115–117.
Niagara Power Vista – visitors center for the Niagara Falls hydro electric plant with displays, a scaled down map of the project, and documentaries on construction, situated atop the cement wall of the plant on the Niagara Gorge.
The Day the Falls Stood Still by Cathy Marie Buchanan
The Whirlpool by Jane Urquhart
Inventing Niagara: Beauty, Power, and Lies by Ginger Strand
Niagara: A History of the Falls Luu tru ngay 23 thang 7 nam 2012 tai Wayback Machine by Pierre Berton
Imagining Niagara: The Meaning and Making of Niagara Falls by Patrick McGreevy