The crash happened around 2:00 a.m. Friday at the westbound I-76 ramp to northbound Route 1 in Philadelphia's East Falls section.
Philadelphia police say a man in his 30s lost control of his motorcycle and was thrown off the bike and then off the twin bridges - plunging 75 feet down into a ravine.
It took rescue crews about 15 minutes before they found him.
The man was secured into a wire basket, brought back up to the scene of the crash. From there he was rushed to Albert Einstein Medical Center in extremely critical condition.
The investigation shut down the ramp for hours, but the ramp has since reopened.
The name of the man injured has not yet been released.