Alloy nanoparticles (NPs), including core-shell, segregated and solid-solution types, show a variety of attractive properties such as catalytic and optical properties and are used in a wide range of applications. Precise control and good reproducibility in the syntheses of alloy NPs are highly demanded because these properties are tunable by controlling alloy structures, compositions, particle sizes, and so on. To improve the efficiency and reproducibility of their syntheses, continuous-flow syntheses with various types of reactors have recently been developed instead of the current mainstream approach, batch syntheses. In this review, we focus on the continuous-flow syntheses of alloy NPs and first overview the flow syntheses of NPs, especially of alloy NPs. Subsequently, the details of flow reactors and their chemistry to synthesize core-shell, segregated, solid-solution types of alloy NPs, and high-entropy alloy NPs are introduced. Finally, the challenges and future perspectives in this field are discussed.