The truck was found completely submerged in water near Puatalefusi wharf.
According to Kaniva News, Neiafu Town Officer Vāvā Lapota said the truck was expected to tow a boat at a launching ramp near before the incident occurred.
It was unclear whether the truck was dragged into the water by the boat or it slid before it plunged into the sea.
Lapota said it was raining and the ramp was slippery.
The boat ramp is notoriously known for its slipperiness and there were boating mishaps there before in which vehicles had been accidentally dragged into the sea, he said.
Photos of the submerged pickup truck and the boat have been shared on social media.
No reports of injuries.
The incident comes after a truck smashed into the MV Taka’ipōmana vessel while it was about to depart for Tongatapu last month.
In August, a road roller overturned and fell into the sea at Vaipua bridge in Neiafu.