The 50-year-old was appointed in February following 11 years at Wycombe.
Ainsworth won only five of his 28 games in charge, and the R’s have now lost six consecutive matches including Saturday’s defeat by Leicester.
Assistant boss Richard Dobson also leaves Rangers, with the club 22nd and with just eight points from 14 games.
“Making a call such as this is never easy, and it is even more difficult when you have such respect on both a personal and professional level for the individuals concerned – which we all do for Gareth and Richard.
Many supporters have told me in recent months that they have never wanted someone to succeed more than Gareth, which is an indication of the fondness everyone associated with QPR has for him.
“Unfortunately, results this season haven’t gone the way we all wanted and we feel a change is necessary.”
The appointment of a new head coach will follow “in the coming days”, Hoos added in his statement.
