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We present the discovery of KELT-21b, a hot Jupiter transiting the $V=10.5$A8V star HD 332124. The planet has an orbital period of$P=3.6127647\pm0.33$ days and a radius of $1.586_{-0.040}^{+0.039}$ $R_J$.We set an upper limit on the planetary mass of $M_P<3.91$ $M_J$ at $3\sigma$confidence. We confirmed the planetary nature of the transiting companion usingthis mass limit and Doppler tomographic observations to verify that thecompanion transits HD 332124. These data also demonstrate that the planetaryorbit is well-aligned with the stellar spin, with a sky-projected spin-orbitmisalignment of $\lambda=-5.6_{-1.9}^{+1.7 \circ}$. The star has$T_{\mathrm{eff}}=7598_{-84}^{+81}$ K, $M_*=1.458_{-0.028}^{+0.029}$$M_{\odot}$, $R_*=1.638\pm0.034$ $R_{\odot}$, and $v\sin I_*=146$ km s$^{-1}$,the highest projected rotation velocity of any star known to host a transitinghot Jupiter. The star also appears to be somewhat metal-poor and$\alpha$-enhanced, with [Fe/H]$=-0.405_{-0.033}^{+0.032}$ and[$\alpha$/Fe]$=0.145 \pm 0.053$; these abundances are unusual, but notextraordinary, for a young star with thin-disk kinematics like KELT-21.High-resolution imaging observations revealed the presence of a pair of stellarcompanions to KELT-21, located at a separation of 1.2" and with a combinedcontrast of $\Delta K_S=6.39 \pm 0.06$ with respect to the primary. Althoughthese companions are most likely physically associated with KELT-21, we cannotconfirm this with our current data. If associated, the candidate companionsKELT-21 B and C would each have masses of $\sim0.12$ $M_{\odot}$, a projectedmutual separation of $\sim20$ AU, and a projected separation of $\sim500$ AUfrom KELT-21. KELT-21b may be one of only a handful of known transiting planetsin hierarchical triple stellar systems.

KELT-21b: A Hot Jupiter Transiting the Rapidly-Rotating Metal-Poor Late-A Primary of a Likely Hierarchical Triple System