This paper presents a novel prototype cone penetrometer for piezocone penetration tests (CPTU). It is equipped with Fibre Optic (FO) sensors for measuring cone resistance (qc ) and sleeve friction (fs ). The FO sensors replace the classical, electric strain-gauges. Field results, laboratory calibration and uncertainty assessment show capability of achieving qc and fs values that are well within Application Class 1 of ISO 22476-1 “electrical cone and piezocone penetration test”. This makes the FO cone penetrometer particularly suitable for use in very soft clays, silts and peat, i.e. Soil A and Interpretation H of ISO 22476-1. The cone penetrometer has potential to meet a significantly more stringent, Dutch Application Class 1+.