Constraints on the intergalactic magnetic field from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>F</mml:mi> <mml:mi>e</mml:mi> <mml:mi>r</mml:mi> <mml:mi>m</mml:mi> <mml:mi>i</mml:mi> </mml:math> -LAT observatio

A cosmological origin of magnetic fields in large scale structures of the Universe would require a non-negligible magnetic field in cosmic voids, which, however, remains undetected. Gamma-ray emission from gamma-ray bursts (GRBs) offers the opportunity to indirectly probe such an intergalactic magnetic field (IGMF), as <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>γ</a:mi> </a:math> rays interact with cosmic radiation fields, producing electron-positron pairs, and initiate an electromagnetic cascade. The deflection of the pairs in the IGMF results in a time-delayed signal at GeV energies. Using observations with the Large Area Telescope of the GRB 221009A, we are able to derive the most stringent constraints to date from the nonobservation of the cascade and rule out magnetic fields <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>B</c:mi> <c:mo>&lt;</c:mo> <c:mn>2.5</c:mn> <c:mo>×</c:mo> <c:msup> <c:mn>10</c:mn> <c:mrow> <c:mo>−</c:mo> <c:mn>17</c:mn> </c:mrow> </c:msup> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi mathvariant="normal">G</c:mi> </c:math> at 95% confidence level for a coherence length larger than 1 Mpc. Our results are comparable to limits obtained from blazar observations but do not suffer from assumptions on the duty cycle of the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mi>γ</f:mi> </f:math> -ray source or whether inverse-Compton scattering losses dominate over the development of plasma instabilities.