Emergency crews responded to Harbor Drive and Y Drive along the waterfront at about 8:30 p.m.
Witnesses said the 4-year-old Muskegon boy came down the hill by the YMCA, went across the street, went down the hill some more, fell off his bike and hit his head on a retaining wall.
Police said the child was unable to stop.
Seventeen-year-old Ben Schultz of Wyoming said the child was halfway over the wall, about to fall into the water, when he grabbed the boy's legs and brought him back to safety.
Schultz said the boy's father ran up and carried the child back across Harbor Drive, where he was met by paramedics and police.
The child suffered a head injury and was transported by ambulance to North Ottawa Community Hospital.
No further information was available on the child's condition.