The sous-vide technique is most commonly used for meat preparation, offering superior nutritional retention, a delicate texture, and enhanced juiciness in the resulting dishes. The purpose of this study was to analyze the effect of time and temperature in sous-vide heat treatment on selected physicochemical properties of horsemeat. Samples of m. longissimus thoracis were heated to 55, 60, 65 °C for 4, 8, 12, and 24 h, and the impact of these heat treatment parameters on chemical composition, physicochemical properties, amount of heme pigments, and color parameters of the tested raw material was determined. Horsemeat subjected to sous-vide heat treatment at 55 °C for 4 h was characterized by a high proportion of red color and oxymyoglobin. Sous-vide treatment carried out at low temperatures and for a short period of time reduces the amount of weight loss and the increase in the TBARS index, allowing the preservation of the red color of the raw material studied. Prolonging the duration or increasing the temperature of sous-vide processing results in a deterioration of the mentioned parameters and also leads to significant changes in the chemical composition of horsemeat, including a significant reduction in the water content and increase in the amount of fat in horsemeat.