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Current theories of planetary evolution predict that infant giant planets have large radii and very low densities before they slowly contract to reach their final size after about several hundred million years1,2. These theoretical expectations remain untested so far as the detection and characterization of very young planets is extremely challenging due to the intense stellar activity of their host stars3,4. Only the recent discoveries of young planetary transiting systems allow initial constraints to be placed on evolutionary models5,6,7. With an estimated age of 20 million years, V1298 Tau is one of the youngest solar-type stars known to host transiting planets; it harbours a system composed of four planets, two Neptune-sized, one Saturn-sized and one Jupiter-sized8,9. Here we report a multi-instrument radial velocity campaign of V1298 Tau, which allowed us to determine the masses of two of the planets in the system. We find that the two outermost giant planets, V1298 Tau b and e (0.64 ± 0.19 and 1.16 ± 0.30 Jupiter masses, respectively), seem to contradict our knowledge of early-stages planetary evolution. According to models, they should reach their mass–radius combination only hundreds of millions of years after formation. This result suggests that giant planets can contract much more quickly than usually assumed. © 2021, The Author(s), under exclusive licence to Springer Nature Limited., A.S.M. acknowledges financial support from the Spanish Ministry of Science and Innovation (MICINN) under the 2019 Juan de la Cierva Programme. J.I.G.H. acknowledges financial support from the Spanish MICINN under the 2013 Ramón y Cajal programme RYC-2013-14875. A.S.M., J.I.G.H., R.R., B.T.-P., N.L., M.R.Z.O., E.G.-A., J.A.C., P.J.A. and I.R. acknowledge financial support from the Spanish Ministry of Science and Innovation through projects AYA2017-86389-P, PID2019-109522GB-C53, PID2019-109522GB-C51, AYA2016-79425-C3-3-P, PID2019-109522GB-C52 and PGC2018-098153-B-C33. M.D. acknowledges financial support from the FP7-SPACE Project ETAEARTH (GA no. 313014). A.M., D.L., G.M., A.S. and S.D. acknowledge partial contribution from the agreement ASI-INAF no. 2018-16-HH.0. S.B., D.L., G.M. and D.T. acknowledge partial contribution from the agreement ASI-INAF no. 2021-5-HH.0. P.J.A. acknowledges financial support from the project SEV-2017-0709. S.D., V.D., S.B. and D.T. acknowledge support from the PRIN-INAF 2019 ‘Planetary systems at young ages’ (PLATEA). D.S.A. thanks the Leverhulme Trust for financial support. I.R. acknowledges the support of the Generalitat de Catalunya/CERCA programme. E.G.-A acknowledges support from the Spanish State Research Agency (AEI) project no. MDM-2017-0737 Unidad de Excelencia ‘María de Maeztu’ Centro de Astrobiología (CAB, CSIC/INTA). D.T. acknowledges the support of the Italian National Institute of Astrophysics (INAF) through the INAF Main Stream project ‘Ariel and the astrochemical link between circumstellar discs and planets’ (CUP: C54I19000700005). E.E.-B. acknowledges financial support from the European Union and the State Agency of Investigation of the Spanish Ministry of Science and Innovation (MICINN) under grant PRE2020-093107 of the Pre-Doc Program for the Training of Doctors (FPI-SO) through FEDER, FSE and FDCAN funds. This work is based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundación Galileo Galilei (FGG) of the Istituto Nazionale di Astrofisica (INAF) at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain). CARMENES is an instrument at the Centro Astronómico Hispano-Alemán (CAHA) at Calar Alto (Almería, Spain), operated jointly by the Junta de Andalucía and the Instituto de Astrofísica de Andalucía (CSIC). This work is based on data obtained with the STELLA robotic telescopes in Tenerife, an AIP facility jointly operated by AIP and IAC. This work makes use of observations from the LCOGT network.