Spent yeast from brewing fermentations is a potential alternative protein source to fishmeal in aquafeed formulations. Using an easily replenished waste stream as a protein replacement would assist with sustainable aquaculture production. Therefore, the primary goal of this study was to assess physical properties of a fishmeal-based aquafeed containing spent brewer’s yeast as a partial protein substitute. This was performed by comparing three pelletized feed formulations containing no yeast, live yeast, and a thermally lysed (dead) yeast. Four primary physical characteristics of the feeds were tested including macronutrient content, texture, water activity, and water stability. For assessment of physical parameters, pellets were dried to ~15% moisture content on a wet basis (MCwb) and ~5% MCwb. Moisture sorption isotherms were practically identical across all three feed formulations, indicating that significant modification of preparatory practices when extruding yeast-modified fish feeds is not needed. Maximum hardness was positively correlated with moisture content and with yeast inclusion. The water stability was lower at lower moisture contents over the course of the first two hours of sampling, but was not correlated after four hours, and decreased with yeast inclusion. Water stability was the lowest for live-yeast inclusive feeds, in which almost all (~90%) solids were lost after 240 min of water exposure. This study examined several key physical effects which incorporating living or dead yeast into an extruded fish feed would be expected to cause, thus forwarding their development as a sustainable protein alternative.