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Full list of authors: Vink, Jacco; Prokhorov, Dmitry; Ferrazzoli, Riccardo; Slane, Patrick; Zhou, Ping; Asakura, Kazunori; Baldini, Luca; Bucciantini, Niccolo; Costa, Enrico; Di Marco, Alessandro; Heyl, Jeremy; Marin, Frederic; Mizuno, Tsunefumi; Ng, C-Y; Pesce-Rollins, Melissa; Ramsey, Brian D.; Rankin, John; Ratheesh, Ajay; Sgro, Carmelo; Soffitta, Paolo; Swartz, Douglas A.; Tamagawa, Toru; Weisskopf, Martin C.; Yang, Yi-Jung; Bellazzini, Ronaldo; Bonino, Raffaella; Cavazzuti, Elisabetta; Costamante, Luigi; Di Lalla, Niccolo; Latronico, Luca; Maldera, Simone; Manfreda, Alberto; Massaro, Francesco; Mitsuishi, Ikuyuki; Omodei, Nicola; Oppedisano, Chiara; Zane, Silvia; Agudo, Ivan; Antonelli, Lucio A.; Bachetti, Matteo; Baumgartner, Wayne H.; Bianchi, Stefano; Bongiorno, Stephen D.; Brez, Alessandro; Capitanio, Fiamma; Castellano, Simone; Ciprini, Stefano; De Rosa, Alessandra; Del Monte, Ettore; Di Gesu, Laura; Donnarumma, Immacolata; Doroshenko, Victor; Dovciak, Michal; Ehlert, Steven R.; Enoto, Teruaki; Evangelista, Yuri; Fabiani, Sergio; Garcia, Javier A.; Gunji, Shuichi; Hayashida, Kiyoshi; Iwakiri, Wataru; Jorstad, Svetlana G.; Karas, Vladimir; Kitaguchi, Takao; Kolodziejczak, Jeffery J.; Krawczynski, Henric; La Monaca, Fabio; Liodakis, Ioannis; Marinucci, Andrea; Marscher, Alan P.; Marshall, Herman L.; Matt, Giorgio; Muleri, Fabio; O'Dell, Stephen L.; Papitto, Alessandro; Pavlov, George G.; Peirson, Abel L.; Perri, Matteo; Pilia, Maura; Possenti, Andrea; Poutanen, Juri; Puccetti, Simonetta; Romani, Roger W.; Spandre, Gloria; Tavecchio, Fabrizio; Taverna, Roberto; Tawara, Yuzuru; Tennant, Allyn F.; Thomas, Nicolas E.; Tombesi, Francesco; Trois, Alessio; Tsygankov, Sergey; Turolla, Roberto; Wu, Kinwah; Xie, Fei.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited., We report on a ∼5σ detection of polarized 3–6 keV X-ray emission from the supernova remnant Cassiopeia A (Cas A) with the Imaging X-ray Polarimetry Explorer (IXPE). The overall polarization degree of 1.8% ± 0.3% is detected by summing over a large region, assuming circular symmetry for the polarization vectors. The measurements imply an average polarization degree for the synchrotron component of ∼2.5%, and close to 5% for the X-ray synchrotron-dominated forward shock region. These numbers are based on an assessment of the thermal and nonthermal radiation contributions, for which we used a detailed spatial-spectral model based on Chandra X-ray data. A pixel-by-pixel search for polarization provides a few tentative detections from discrete regions at the ∼ 3σ confidence level. Given the number of pixels, the significance is insufficient to claim a detection for individual pixels, but implies considerable turbulence on scales smaller than the angular resolution. Cas A's X-ray continuum emission is dominated by synchrotron radiation from regions within ≲1017 cm of the forward and reverse shocks. We find that (i) the measured polarization angle corresponds to a radially oriented magnetic field, similar to what has been inferred from radio observations; (ii) the X-ray polarization degree is lower than in the radio band (∼5%). Since shock compression should impose a tangential magnetic-field structure, the IXPE results imply that magnetic fields are reoriented within ∼1017 cm of the shock. If the magnetic-field alignment is due to locally enhanced acceleration near quasi-parallel shocks, the preferred X-ray polarization angle suggests a size of 3 × 1016 cm for cells with radial magnetic fields. © 2022. The Author(s). Published by the American Astronomical Society., The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC), and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. This research used data products provided by the IXPE Team (MSFC, SSDC, INAF, and INFN) and distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center (HEASARC), at NASA Goddard Space Flight Center (GSFC). J.V. and D.P. are supported by funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 101004131 (SHARP). The research at Boston University was supported in part by National Science Foundation grant AST-2108622. We thank Dawoon Kim for kindly providing us with his script for making polar plots of polarization degree and angle., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.