Frith, who was appointed editor of the celebrity weekly title in 2000, is leaving in May to a write a book.
Julian Linley, editor of heatworld.com, will be acting editor until a replacement for Frith is confirmed, a spokesman for Bauer said.
Linley will continue his role as editor of the website, at the same time as being acting editor.
Frith started his magazine career in 1990, when he joined Emap-owned Smash Hits. He was appointed editor of the title in 1994, at the age of 23, before moving to Sky magazine in 1996.
During his tenure, Heat's circulation has increased from 65,000 to more than 500,000.
The magazine's circulation is 533,034, according to the latest ABC figure reported earlier this month, a fall of 11 per cent year on year.
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