The reduced phosphomolybdate {P₄Mo₆X₃₁}ⁿ⁻ (X = O or OH) ({P₄Mo₆}) is an important member of polyoxometalates (POMs), characterized by fully reduced Mo atoms, multiple oxygen coordination sites, and excellent optical response and electrochemical properties, which have attracted extensive interest from researchers. Over the past few decades, the development of {P₄Mo₆}-based POMs has been relatively slow compared to classical polyoxometalates. Most studies have primarily focused on synthesizing novel structures and some investigations into properties such as photocatalytic degradation of organic dyes, and electrocatalytic reduction of hydrogen peroxide. Recently, {P₄Mo₆}-based POMs have attracted increasing interest owing to their unique structural advantages and potential applications in photocatalysis, electrocatalysis, electrochemical sensors, and proton conduction. This review summarizes the development of {P₄Mo₆}-based POMs over the past decade, focusing on their structures, synthesis, and applications across various fields, as well as the use of {P₄Mo₆}-based POMs as precursors in nanocomposites and their applications. Despite these advances, challenges remain, such as improving the scalability of {P₄Mo₆}-based POMs and intensively studying the relationship between structures and performance. Future research will likely explore potential energy storage and conversion applications, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and battery materials. This review aims to provide researchers with a comprehensive understanding of this area and to guide future developments of {P₄Mo₆}-based POMs.