The relativistic linear momentum of a body of mass m and speed v is

*Note: Bold letters are designated as vectors.

p = mv / √[ 1 - (v/c)2 ] = γmv

where γ = 1 / √[ 1 - (v/c)2 ] and γ > 1. Some prefer to associate the γ with the mass and introduce a relativistic mass mR = γm.. That allows you to write the momentum as p = mRv, but mR is speed dependent. Usually one mass m is used, which is independent of speed, just like the two other fundamental properties of particles of mattter, charge and spin.