Daphne Caruana Galizia, 53, was reportedly killed when the car she was driving exploded shortly after she left her home in Bidnija, near Mosta.
Local media say one of her sons heard the blast and rushed outside.
PM Joseph Muscat, whom Caruana Galizia accused of wrongdoing earlier this year, denounced the killing.
"I condemn without reservations this barbaric attack on a person and on the freedom of expression in our country," he said in a televised statement.
"Everyone knows Ms Caruana Galizia was a harsh critic of mine, both politically and personally, as she was for others too."
But he stressed there could be "no justification... in any way" for such action.
"I will not rest before justice is done."
On Monday evening, thousands of people attended a candlelit vigil in the resort town of Sliema.
Malta Television reported that Caruana Galizia had filed a complaint to the police two weeks ago to say she had received threats but gave no further information.
Police have opened a murder inquiry.