Van Landeghem claimed Canada's first swimming medal of the Games in 53.83 seconds to set a new Pan Am record.
Coughlin took silver in 54.06 seconds.
Arianna Vanderpool-Wallace of the Bahamas finished third in 54.15.
Canada added another gold when Montreal's Audrey Lacroix won the women's 200-metre butterfly in two minutes 7.68 seconds.
Katherine Mills of the United States finished second in 2:09.31 and Joanna Maranhao was third in 2:09.38.
In the men's 100-metre freestyle, Santo Condorelli of Kenora, Ont., won the silver medal in 48.57 seconds.
Federico Grabich of Argentina won gold in 48.26.