The exact amount of the reward was not specified by the AXA Art insurance company but it was described as substantial.
An additional $10,000 is being offered to anyone who can identify a man caught on the museum's closed-circuit security cameras.
Video shown to police and insurance investigators clearly shows the robber carrying out the theft in the museum gallery where the pieces were on display.
The pieces — an Assyrian bas-relief and a marble head dating from the Roman Empire — are together valued in the hundreds of thousands of dollars.
They were taken last fall but the theft was not made public to protect a police investigation.
The museum says the theft is an isolated incident and that items are protected by security systems that include motion detectors.
Guards also patrol the galleries.