We demonstrate a giant Rashba-type spin splitting on a semiconducting substrate by means of a Bi-trimer adlayer on a Si(111) wafer. The in-plane inversion symmetry is broken including a giant spin splitting with a Rashba energy of about 140meV, much larger than what has prviously been reported for any semiconductor heterostructure. The separation of the electronic states is larger than their lifetime broadening, which has been directly observed with angular resolved photomission spectroscopy. The experimental results are confirmed by relativistic first-principles calculations.