The definable fundamental group of a definable set in an $o$ -minimal expansion of a field is computed. This is achieved by proving the relevant case of the $o$ -minimal van Kampen theorem. This result is applied to show that if the geometrical realization of a simplicial complex over an $o$ -minimal expansion of a field is a definable manifold of dimension not 4, then its geometrical realization over the reals is a topological manifold.