If heavy neutrinos with mass mνH≥2me are produced in the Sun via the decay 8B→8Be+e++νH in a side branch of the pp chain, they would undergo the observable decay into an electron, a positron and a light neutrino νH→νL+e++e−. In the present work Borexino data are used to set a bound on the existence of such decays. We constrain the mixing of a heavy neutrino with mass 1.5 MeV≤mνH≤14 MeV to be |UeH|2≤(10−3−4×10−6), respectively. These are tighter limits on the mixing parameters than obtained in previous experiments at nuclear reactors and accelerators.