Protein post-translational modification (PTM) by ubiquitination has been observed during many aspects of plant growth, development, and stress responses. The ubiquitin-proteasome system precisely regulates phytohormone signaling by affecting protein activity, localization, assembly, and interaction ability. Abscisic acid (ABA) is a major phytohormone, and it plays important roles in plants under standard or stressed growth conditions. Multiple regulations of ABA signaling transducers by ubiquitination have attracted our attention. Particularly, recent studies have identified different types of E3 ligases that mediate ubiquitination of ABA receptors in different cell compartments. The ABA signaling pathway is composed of phosphatases, kinases, transcription factors, and membrane ion channels. This review focuses on modulation of these components by monoubiquitination or polyubiquitination that occurs in the plasma membrane, endomembranes, and from the cytosol to the nucleus; this implies the existence of retrograde and trafficking processes in ABA signaling by ubiquitination. A number of single-unit E3 ligases, components of multi-subunit E3 ligases, E2s, and specific subunits of the 26S proteasome involved in ABA signal regulation have been discussed. The precise functions of ubiquitination in the ABA pathway may help us understand key factors in the signaling of other phytohormones regulated by ubiquitination or other types of PTMs.