A coolant heater for a vehicle includes: a housing unit having an inlet part through which a coolant is introduced and an outlet part through which the coolant is discharged; a baffle assembly provided in an internal space of the housing unit and having a first flow path through which the coolant flows in a first direction and a second flow path through which the coolant, passing through the first flow path, flows in a second direction different from the first direction; a first heater part provided in the first flow path; and a second heater part provided in the second flow path. The coolant heater makes it possible to obtain an advantageous effect of improving a fast-acting heating performance and heating efficiency.