Hamilton could get up to 25 millimeters of rain tonight and early Thursday.
Environment Canada issued a special weather statement to most of south western Ontario warning residents of the significantly damp evening.
The area of rain is expected to blanket the southern part of the province from west to east, then taper off Thursday morning.
Overnight temperatures are expected to be as high as 4 C.
Conditions overnight are expected to bring foggy patches to the area.
Heavy rain could cause dangerous conditions near creeks.