No. Rat snakes and king snakes are different groups of nonvenomous colubrid snakes, not the same species or genus. In North America, many familiar ratsnakes are placed in the genus Pantherophis (for example, the gray ratsnake, Pantherophis spiloides), while kingsnakes belong to the genus Lampropeltis (for example, the common kingsnake, Lampropeltis getula).
U.S. Geological Survey species datasets treat these lineages separately—publishing distinct range products for ratsnakes such as the gray ratsnake and for kingsnakes such as the common kingsnake—so they are not interchangeable names for one animal. Both groups can look dark or patterned and may overlap in some habitats, which is why people sometimes confuse them.
Source: U.S. Geological Survey — Gray Ratsnake (Pantherophis spiloides) range map (see also USGS Common Kingsnake range data for Lampropeltis getula).