In this paper we determine the constants in multivariate Markov inequalities in the L2-norm on an interval, a triangle and a tetrahedron. Using orthonormal polynomials, we derive explicit expression for the constants on that given 1-simplex, 2-simplex and 3-simplex. Accurate values for the constants are crucial for the correct derivation of a priori and a posteriori error estimations in adaptive computation.