Chief inspector Christine Gilbert is leaving Ofsted at the end of June - months before her contract was due to come to an end.
Ms Gilbert has headed the watchdog since 2006, but the Department for Education announced this week that she will be leaving this summer. Her contract was due to expire in October, and it had previously been made public that it would not be renewed, but the early departure comes as a surprise.
Ms Gilbert has been rumoured to have had a difficult relationship with education secretary Michael Gove.
In her resignation letter, she said the departure date "allows me to take the three-month break necessary between leaving a crown servant post and taking up alternative employment".
An Ofsted spokesman declined to comment on whether Ms Gilbert had been offered a new job elsewhere.
Ms Gilbert oversaw the watchdog's expansion in 2007 when it took on the role of four other inspectorates. As well as inspecting schools, its wider remit includes childcare, children's social care, education and adult learning.
Ofsted chair Baroness Sally Morgan said: "Christine Gilbert will be a hard act to follow. She can be justifiably proud of her achievements with Ofsted which she leaves stronger and ready to face the challenges ahead."
In his letter accepting Ms Gilbert's resignation, Mr Gove said: "You have been an outstanding public servant and someone whom it has been a pleasure to work with."