Brewers’ spent grain (BSG) composes the largest portion of waste produced during beer brewing. At current, it is usually discarded to landfill or used as livestock feed. However, emerging research suggests it may be a viable functional food ingredient to be used in human foodstuff due to its nutritional properties. Diminishing resources and climate change are putting pressure on agriculture to ensure food security; thus, the food industry has an increasing interest in novel low-cost sources of functional food ingredients. This is further driven by consumers demanding healthy food products and the growing “healthy snack” food category. Muesli bar samples were produced with four different percentages of BSG to oats: 0% (control), 15%, 25%, and 50%. Colour and texture measurements were taken to assess the end-product quality. Total starch, dietary fibre, ash, and protein were measured to determine the nutritional content of the muesli bars. The addition of BSG in muesli bars had a significant effect on colour. BSG caused the muesli bars to increase in darkness and redness, but there was also a decrease in yellowness. No clear trend or significant difference was determined when comparing the hardness of the muesli bars. The addition of BSG in muesli bars had no significant effect (p < 0.05) on the level of starch, protein, and ash contents. However, dietary fibre significantly increased in the muesli bars with the addition of BSG (p < 0.05). These results show that BSG does have a significant positive effect on muesli bar nutrition by providing higher levels of protein and a significantly greater amount of dietary fibre. This is of great interest to the food industry due to their interest in upcycled foods as a novel low-cost source of food ingredients.