"The club can announce this evening that it has parted company with head coach Mike Phelan," Hull said on Twitter. "The search for a replacement has begun and an announcement will be made in due course."
The 54-year-old Phelan was only appointed as Hull boss on a permanent basis in October but Monday's 3-1 loss at West Brom left the club with just 13 points from 20 games.
"If we carry on that way, weâre going to get punished in every game," Phelan told the Hull website after the latest loss.
"Playing well is irrelevant if you canât do the basics. We have to learn and grow quickly. We canât gift teams the opportunities that we do."