A bicyclist was fatally struck by an MTA bus in Manhattan Tuesday morning, cops confirmed.

The unidentified woman was riding her bike at Fifth Avenue and East 59th Street just before 9 a.m. when the BxM10 bus hit her, police said.

The woman and the MTA bus were traveling in the same direction, with the bicyclist ahead — and authorities are probing how the smash-up happened, according to cops.

Citing preliminary information, an MTA source said a female bicyclist “made contact” with the right rear of the southbound bus. The source called the incident an “active investigation.”

The bus was occupied, but no one on board was initially aware the woman was struck, police said.

The bus driver stayed on scene, and no one else was hurt, police said.