Global <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi mathvariant="normal">Λ</mml:mi></mml:math>-hyperon polarization in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Au</mml:mi><mml:mo>+</mml:mo><mml:mi>Au</mml:mi></mml:mrow></mml:math> collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mi>NN</mml:mi></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>3</mml:mn><mml:mo> </mml:mo><mml:mi>GeV</mml:mi></mml:mrow></mml:math>

Global hyperon polarization, ${\overline{P}}_{\mathrm{H}}$, in $\mathrm{Au}+\mathrm{Au}$ collisions over a large range of collision energy, $\sqrt{{s}_{\mathrm{NN}}}$, was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While na\"{\i}ve extrapolation of data trends suggests a large ${\overline{P}}_{\mathrm{H}}$ as the collision energy is reduced, the behavior of ${\overline{P}}_{\mathrm{H}}$ at small $\sqrt{{s}_{\mathrm{NN}}}&lt;7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of $\mathrm{\ensuremath{\Lambda}}$ hyperons along the direction of global angular momentum in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathrm{NN}}}=3$ GeV. The observation of substantial polarization of $4.91\ifmmode\pm\else\textpm\fi{}0.81(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.15(\mathrm{syst}.)%$ in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.