jats:titleAbstract</jats:title>jats:pThe structure and function of biomolecular condensates are closely related. However, many studies and applications of this relationship are prevented because controlling the mesoscale architecture of condensates can be difficult. Here we introduce a way to create custom multiphase architectures by nucleating new droplets in condensates. This nucleation occurs due to limited diffusion in the dense condensates and a composition change forced upon the system by changing the experimental conditions. The designed architectures are transient states created out of equilibrium. We provide a detailed method for understanding and designing a range of condensate architectures. Access to these long-lived complex architectures will enable researchers to incorporate increasingly sophisticated compartmentalization and functionality in condensates. This general strategy for creating complex structured condensates out of equilibrium may also provide insights into the structure of condensates in cells.</jats:p>