jats:titleAbstract</jats:title> jats:pWe present new observations with the Atacama Large Millimeter/submillimeter Array for a gravitationally lensed galaxy at jats:italicz</jats:italic> = 9.1, MACS1149-JD1. [O jats:sciii</jats:sc>] 88 jats:italicμ</jats:italic>m emission is detected at 10jats:italicσ</jats:italic> with a spatial resolution of ∼0.3 kpc in the source plane, enabling the most distant morphokinematic study of a galaxy. The [O jats:sciii</jats:sc>] emission is distributed smoothly without any resolved clumps and shows a clear velocity gradient with Δjats:italicV</jats:italic> jats:subobs</jats:sub>/2jats:italicσ</jats:italic> jats:subtot</jats:sub> = 0.84 ± 0.23, where Δjats:italicV</jats:italic> jats:subobs</jats:sub> is the observed maximum velocity difference and jats:italicσ</jats:italic> jats:subtot</jats:sub> is the velocity dispersion measured in the spatially integrated line profile, suggesting a rotating system. Assuming a geometrically thin self-gravitating rotation disk model, we obtain jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msub mml:mrow mml:miV</mml:mi> </mml:mrow> mml:mrow mml:mirot</mml:mi> </mml:mrow> </mml:msub> mml:mrow <mml:mo stretchy="true">/</mml:mo> </mml:mrow> mml:msub mml:mrow mml:miσ</mml:mi> </mml:mrow> mml:mrow mml:miV</mml:mi> </mml:mrow> </mml:msub> mml:mo=</mml:mo> mml:msubsup mml:mrow mml:mn0.67</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn0.26</mml:mn> </mml:mrow> mml:mrow mml:mo+</mml:mo> mml:mn0.73</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac7447ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, where jats:italicV</jats:italic> jats:subrot</jats:sub> and jats:italicσ</jats:italic> jats:sub jats:italicV</jats:italic> </jats:sub> are the rotation velocity and velocity dispersion, respectively, still consistent with rotation. The resulting disk mass 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:mn0.65</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn0.40</mml:mn> </mml:mrow> mml:mrow mml:mo+</mml:mo> mml:mn1.37</mml:mn> </mml:mrow> </mml:msubsup> mml:mo×</mml:mo> mml:msup mml:mrow mml:mn10</mml:mn> </mml:mrow> mml:mrow mml:mn9</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac7447ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> jats:italicM</jats:italic> jats:sub⊙</jats:sub> is consistent with being associated with the stellar mass identified with a 300 Myr old stellar population independently indicated by a Balmer break in the spectral energy distribution. We conclude that the most of the dynamical mass is associated with the previously identified mature stellar population that formed at jats:italicz</jats:italic> ∼ 15.</jats:p>