Metamaterials offer enormous opportunities and unprecedented functionalities to manipulate electromagnetic waves enabling promising applications such as invisibility cloaking, superfocusing and subwavelength confinement. The exotic electromagnetic behavior of metamaterials was dramatically empowered by dynamic control through incorporation of active media. A prominent example is a graphene metamaterial – an integration of a two-dimensional monolayer graphene with a planar metasurface, where the unique optical and electronic properties of graphene is inherited by the metamaterial, thus opening up fascinating possibilities in electromagnetic wave control. Here, through a combination of continuous wave (CW) optical illumination and electrical gating, we demonstrate a giant active modulation of terahertz waves in a graphene–silicon hybrid metamaterial at extremely low bias voltages. The highly tunable characteristics of the graphene metamaterial device under electrical bias and optical illumination open up new avenues for graphene-based high performance integrated active photonic devices compatible with the silicon technology.