This dissertation presents a search for Planck-scale mass dark matter with SNO+, a large liquid scintillator neutrino experiment. It begins with an overview of dark matter, summarizing its astrophysical and cosmological evidence, discussing two leading candidates—Weakly Interacting Massive Particle (WIMP) and Multiply Interacting Massive Particle (MIMP), with a particular focus on the latter—and reviewing detection strategies, especially direct detection experiments. An overview of the SNO+ experiment is then provided, along with a detailed description of its detector hardware and software, highlighting the energy calibration and systematic uncertainty evaluation using a $^{16}$N source during the water phase. Afterwards, a detailed study of MIMP propagation through the earth's overburden and the liquid scintillator for accurate simulation is presented, along with the expected signatures for a MIMP with mass $m_{\chi} = 10^{19}$~GeV\footnote{This work uses natural units where $\hbar = c = 1.$} and spin-independent nucleon cross section $\sigma^{\text{SI}}_{\chi n} = 5\times10^{-27}$~cm$^{2}$. Relevant backgrounds, event tagging methods, and systematic uncertainties are also discussed. Over 134.4 live days of data, two candidate MIMP events were identified. Further investigation suggests these events likely originated from the detector neck, a background source not included in the original model. As a result, no statistically significant conclusion could be drawn for the MIMP model probed. Assuming no backgrounds are expected, dark matter models that predict 5.32 observed events in SNO+ can be excluded with these 2 observed events with 90% confidence level. Finally, the live time required to exclude the MIMP of interest at the 90% confidence level is projected, and several suggestions for improving tagging efficiency in future studies are proposed.