Whey, a large by-product of cheese manufacturing, is nutritionally valuable, mainly because of its whey proteins. Beetroot juice, purple–red in colour, has numerous benefits for human health; if used as an ingredient in a whey-based RTD beverage, it could mask whey’s less pleasant organoleptic characteristics. This research aimed to develop such a beverage by mixing whey with beetroot juice in a ratio that would yield a nutritionally valuable product with acceptable sensory characteristics. To this end, three RTD beverage formulations, prepared with beetroot juice and whey in different ratios (10:90, 15:85, and 20:80 v/v), were subjected to sensory analysis. The one with a 20:80 v/v ratio of beetroot juice and whey was found to be preferred by consumers, registering the highest overall score (7.8). Next, this formula was prepared with (RTD20:80a) and without citric acid (RTD20:80a) and analysed physiochemically and microbiologically. RTD20:80a was also sensory tested and proved to be the optimum one (with an overall score of 8.1); it showed a content of 1.18 g/100 mL fat, 1.07 g/100 mL protein, 0.56 g/100 mL ash, 9.90 g/100 mL carbohydrate, total soluble solids of 11.69 °Brix, an energy value of 54.44 kcal/100 mL, titratable acidity of 0.77% lactic acid, pH of 3.75, content of 1.16 mg/mL betacyanins, 0.86 mg/mL betaxanthins, and 0.36 mg GAE/mL total phenolics. Based on the microbiological results, RTD20:80a revealed a shelf-life of 14 days. By producing this beverage, a dairy plant could reduce its whey-related losses while increasing its profitability through selling it.