The accident happened in the parking lot of an apartment complex at 4542 Astoria Lane in Taylorsville.
Unified police said the 48-year-old father was riding his motorcycle with his infant son between him and the motor. Neither were wearing a helmet.
Witnesses said the man was driving fast. He hit the brakes on a turn and crashed. The boy was tossed from the bike and killed.
Betty Reiz watched the wreck unfold. She said the baby tried to stand up after the accident, but fell down and died.
"The father is always with the child in front of the motorcycle," Reiz said.
She said she felt guilty for not stepping in.
Unified Police Detective Levi Hughes said the death should have never happened.
"This was a tragic accident," Hughes said. "It's a situation that common sense would have helped us avoid today."
The father may face criminal charges for his son's death pending the outcome of the investigation.