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This paper presents improved constraints on the low-mass stellar initial massfunction (IMF) of the Bo\"otes I (Boo~I) ultrafaint dwarf galaxy, based on ouranalysis of recent deep imaging from the Hubble Space Telescope. Theidentification of candidate stellar members of Boo~I in the photometric catalogproduced from these data was achieved using a Bayesian approach, informed bycomplementary archival imaging data for the Hubble Ultra Deep Field.Additionally, the existence of earlier-epoch data for the fields in Boo~Iallowed us to derive proper motions for a subset of the sources and thusidentify and remove likely Milky Way stars. We were also able to determine theabsolute proper motion of Boo~I, and our result is in agreement with, butcompletely independent of, the measurement(s) by \textit{Gaia}. Thebest-fitting parameter values of three different forms of the low-mass IMF werethen obtained through forward modeling of the color-magnitude data for likelyBoo~I member stars within an approximate Bayesian computation Markov chainMonte Carlo algorithm. The best-fitting single power-law IMF slope is $\alpha =-1.95_{-0.28}^{+0.32}$, while the best-fitting broken power-law slopes are$\alpha_1 = -1.67_{-0.57}^{+0.48}$ and $\alpha_2 = -2.57_{-1.04}^{+0.93}$. Thebest-fitting lognormal characteristic mass and width parameters are$\rm{M}_{\rm{c}} = 0.17_{-0.11}^{+0.05} \cal M_\odot$ and$\sigma=0.49_{-0.20}^{+0.13}$. These broken power-law and lognormal IMFparameters for Boo~I are consistent with published results for the stars withinthe Milky Way and thus it is plausible that Bo{\"o}tes I and the Milky Way arepopulated by the same stellar IMF.

Faint Stars in a Faint Galaxy: II. The Low Mass Stellar Initial Mass Function of the Bo\"otes I Ultrafaint Dwarf Spheroidal Galaxy