Bystanders watched in horror Thursday afternoon as the girl was tossed into the water, WFTS-TV reports.
Witnesses told police a woman carried the girl to the edge of a bridge before throwing her off. However, police said the woman, identified as 26-year-old Shakayla Denson, waded into the river with the girl, tossed her into the water, and walked away.
The little girl was identified as Je'Hyrah Daniels.
A police dive team was called to the scene. Officers found the unconscious girl submerged about 75 feet off the shore. They loaded her into an ambulance and rushed her to the hospital, but it was already too late.
The Tampa Police Department says Denson was located walking not far from the scene where all of this happened.
Details on a possible motive were not immediately available.
News outlets reported that Denson was denied bail Friday during her first court appearance on first-degree murder charges. She was also charged with child abuse and auto theft. Authorities said Denson drove Je'Hyrah to the river in a stolen vehicle.
In 2015, police say a man threw his 5-year-old daughter to her death off the Skyway Bridge in St. Petersburg. John Jonchuck Jr. is set for trial next month.
The Associated Press and KTRK-TV contributed to this report.