The warning was issued on Tuesday afternoon and is valid between 8.00am on Wednesday and 11:59pm on Friday.
Temperatures in some parts are expected to reach as high as 30°C.
"The current hot weather is expected to continue until later this week, with daytime temperatures peaking on Wednesday and Thursday.
"High overnight temperatures are also expected before temperatures fall on Friday," said the Met Office
The extreme heat warning applies to all of Northern Ireland.