With almost all votes counted, Kast had support of about 58 percent.
It was enough for the left coalition and candidate Jeannette Jara to admit defeat.
Kast is described as an ultra-conservative and the most right-leaning presidential candidate since the brutal dictator Augusto Pinochet left power. Kast has also defended Pinochet.
Among other things, Kast has gone to the polls to deport more than 300,000 immigrants and close the border in the north.
