Abstract: Multi-messenger astrophysics is a powerful tool for understanding the most energetic sources in the universe. Although IceCube has discovered a flux of extragalactic neutrinos, the sources of the vast majority of those neutrinos remain a mystery. Several classes of astrophysical transients are proposed sources of neutrino emission. By searching for neutrinos from these sources in realtime, we can identify multi-messenger sources rapidly to inform follow-up by other telescopes. In this thesis, a broad range of transients are analyzed, including gravitational wave transients, nearby supernovae, bright gamma-ray bursts, and novae. Compact object mergers, detected by the LIGO-Virgo-KAGRA detectors, may produce neutrinos alongside their gravitational wave emission. The hundreds of times smaller localization area of neutrino track events can inform follow-up by other observatories in realtime from these sources, increasing the probability of identifying the multi-messenger source. Another exciting transient event which occurred in October 2022 was the brightest gamma-ray burst ever recorded, GRB 221009A. We search for neutrinos from this GRB in realtime and in an archival search leveraging the full energy capabilities of the detector. We set the strongest constraints on neutrino emission from GRBs using this single burst, due to its bright electromagnetic flux and the IceCube non-detection. Finally, we search for sub-TeV neutrinos from nearby and bright novae in both archival data and in realtime. In archival data, we search for neutrinos from the nova RS Ophiuchi in its 2021 outburst, the only nova so far detected to TeV gamma-ray energies. This analysis can be applied to the upcoming eruption of nova T Coronae Borealis, which is both closer and brighter in optical flux than RS Oph, meaning the expected neutrino emission is several times higher than that of RS Oph. IceCube will be able to search for neutrino emission from this nova in realtime using two neutrino datasets spanning GeV-PeV neutrino energies.