The Tanker Trackers site also shows the Adrian Darya 1 near Tartus
"Anyone who said the Adrian Darya-1 wasn't headed to Syria is in denial," Bolton said in a tweet late Friday with a satellite image he said showed the ship anchored two nautical miles off Tartus.
"Tehran thinks it's more important to fund the murderous Assad regime than provide for its own people. We can talk, but Iran's not getting any sanctions relief until it stops lying and spreading terror!"
The Tanker Trackers site also shows the Adrian Darya 1 near Tartus.
However there was no confirmation that the ship, carrying 2.1 million barrels of oil worth around $140 million, was unloading its cargo.
The Middle East Eye news site said Friday the tanker had delivered oil to Syria, with one source quoted saying 55% of its cargo was offloaded Thursday night.
The Adrian Darya 1 had been held for six weeks by the British overseas territory of Gibraltar on suspicion it was set to deliver oil from Iran to its main Arab ally Syria - a violation of European Union sanctions on President Bashar al-Assad's iron-fisted regime.
Gibraltar released the ship, formerly called the Grace 1, on August 18 over US protests after receiving written assurances that the vessel would not head to countries under European Union sanctions.
Tehran later denied it had made any promises about the destination of the ship, which had been elusive since leaving Gibraltar.
Tensions between arch-enemies Iran and the US have soared since May last year when President Donald Trump pulled out of a landmark 2015 nuclear accord between Tehran and major powers, and began reimposing sanctions that have crippled its economy.
European governments have declined to follow the Trump administration's tough line against Tehran, instead seeking to salvage the nuclear deal by finding a mechanism to get round US sanctions.
But both the EU and the US have adopted a raft of sanctions against Syria, including an oil embargo.