  Hexagonal boron nitride (hBN) is a van der Waals material with excellent
mechanical properties hosting quantum emitters and optically active spin
defects, several of them being sensitive to strain. Establishing optomechanical
control of hBN will enable hybrid quantum devices that combine the spin degree
of freedom with the cavity optomechanical toolbox. In this letter, we report
the first observation of radiation pressure backaction at telecom wavelengths
with a hBN drum-head mechanical resonator. The thermomechanical motion of the
resonator is coupled to the optical mode of a high finesse fiber-based
Fabry-P\'erot microcavity in a membrane-in-the-middle configuration. We are
able to resolve the optical spring effect and optomechanical damping with a
single photon coupling strength of $g_0/2\pi = 1200$ Hz. Our results pave the
way for tailoring the mechanical properties of hBN resonators with light.
