The coupled semi-discrete modified Korteweg-de Vries equation in (2+1)-dimensions is proposed. It is shown that it can be decomposed into two (1+1)-dimensional differential-difference equations belonging to mKdV lattice hierarchy by considering a discrete isospectral problem. A Darboux transformation is set up for the resulting (2+1)- dimensional lattice soliton equation with the help of gauge transformations of Lax pairs. As an illustration by example,the soliton solutions of the mKdV lattice equation in (2+1)-dimensions are explicitly given.