The man, aged in his 50s, was hit by a National Express bus on the A459 Wolverhampton Road in Sedgley at around 8.20am on Saturday.
The road was cordoned off and shut throughout the morning as collision investigators examined the scene, while the man remained in a serious condition in hospital on Sunday.
The collision happened near Wolverhampton Road's junction with High Park Crescent.
An air ambulance was sent to the incident and landed on the playing field of nearby Dormston School, but the man is understood to have been taken to hospital by land ambulance.
Members of the public are believed to have rushed to the man's aid following the incident.
A woman, who lives on Wolverhampton Road, said: "This road is always noisy. We knew something had happened when everyone was turning around."
A West Midlands Police spokesman asked anyone with information to contact police via Live Chat at west-midlands.police.uk or by calling 101 and quoting log number 624.
A National Express West Midlands spokesman said: "A National Express West Midlands bus on service one was involved in an incident at about 8.15am at Wolverhampton Road/Gibbens Hill.
"Emergency services were called and we will be giving them every assistance in their investigations into what happened.
"We'd like to apologise to anyone whose journey was delayed by this incident."