jats:titleAbstract</jats:title> jats:pWe present new measurements of cosmic microwave background (CMB) lensing over 9400 degjats:sup2</jats:sup> of the sky. These lensing measurements are derived from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) CMB data set, which consists of five seasons of ACT CMB temperature and polarization observations. We determine the amplitude of the CMB lensing power spectrum at 2.3% precision (43jats:italicσ</jats:italic> significance) using a novel pipeline that minimizes sensitivity to foregrounds and to noise properties. To ensure that our results are robust, we analyze an extensive set of null tests, consistency tests, and systematic error estimates and employ a blinded analysis framework. Our CMB lensing power spectrum measurement provides constraints on the amplitude of cosmic structure that do not depend on Planck or galaxy survey data, thus giving independent information about large-scale structure growth and potential tensions in structure measurements. The baseline spectrum is well fit by a lensing amplitude of jats:italicA</jats:italic> jats:sublens</jats:sub> = 1.013 ± 0.023 relative to the Planck 2018 CMB power spectra best-fit ΛCDM model and jats:italicA</jats:italic> jats:sublens</jats:sub> = 1.005 ± 0.023 relative to the ACT DR4 + WMAP best-fit model. From our lensing power spectrum measurement, we derive constraints on the parameter combination jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msubsup mml:mrow mml:miS</mml:mi> </mml:mrow> mml:mrow mml:mn8</mml:mn> </mml:mrow> mml:mrow mml:miCMBL</mml:mi> </mml:mrow> </mml:msubsup> mml:mo≡</mml:mo> mml:msub mml:mrow mml:miσ</mml:mi> </mml:mrow> mml:mrow mml:mn8</mml:mn> </mml:mrow> </mml:msub> mml:msup mml:mrow <mml:mfenced close=")" open="("> mml:mrow mml:msub mml:mrow <mml:mi mathvariant="normal">Ω</mml:mi> </mml:mrow> mml:mrow mml:mim</mml:mi> </mml:mrow> </mml:msub> mml:mrow <mml:mo stretchy="true">/</mml:mo> </mml:mrow> mml:mn0.3</mml:mn> </mml:mrow> </mml:mfenced> </mml:mrow> mml:mrow mml:mn0.25</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjacfe06ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> of jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msubsup mml:mrow mml:miS</mml:mi> </mml:mrow> mml:mrow mml:mn8</mml:mn> </mml:mrow> mml:mrow mml:miCMBL</mml:mi> </mml:mrow> </mml:msubsup> mml:mo=</mml:mo> mml:mn0.818</mml:mn> mml:mo±</mml:mo> mml:mn0.022</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjacfe06ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> from ACT DR6 CMB lensing alone and jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msubsup mml:mrow mml:miS</mml:mi> </mml:mrow> mml:mrow mml:mn8</mml:mn> </mml:mrow> mml:mrow mml:miCMBL</mml:mi> </mml:mrow> </mml:msubsup> mml:mo=</mml:mo> mml:mn0.813</mml:mn> mml:mo±</mml:mo> mml:mn0.018</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjacfe06ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> when combining ACT DR6 and Planck jats:monospaceNPIPE</jats:monospace> CMB lensing power spectra. These results are in excellent agreement with ΛCDM model constraints from Planck or ACT DR4 + WMAP CMB power spectrum measurements. Our lensing measurements from redshifts jats:italicz</jats:italic> ∼ 0.5–5 are thus fully consistent with ΛCDM structure growth predictions based on CMB anisotropies probing primarily jats:italicz</jats:italic> ∼ 1100. We find no evidence for a suppression of the amplitude of cosmic structure at low redshifts.</jats:p>