<p>The manufacturing of fruit products without additives has attracted growing interest in response to consumer preferences for natural, minimally processed, and clean-label foods. Although blackthorn (<i>Prunus spinosa</i> L.) has been investigated for its bioactive potential, limited information is available regarding the effects of traditional thermal processing on additive-free blackthorn syrup. This study aimed to investigate the effect of traditional heat treatment applied during the production of additive-free blackthorn syrup on its physicochemical properties, phenolic compound profile, and antioxidant activity. Analyses were performed on raw fruit, fruit marc, filtered juice, and syrup samples collected at different times (0, 30, 60, 90, and 120&#xa0;min) during the boiling stage. Physicochemical analyses showed that boiling progressively concentrated the syrup (13.30 to 51.20 °Brix), increased titratable acidity (1.25% to 4.89%), decreased water activity (0.981 to 0.903), and induced color changes due to pigment degradation and non-enzymatic browning. Early stages of thermal processing promoted the release of bound phenolic compounds and enhanced antioxidant capacity through cell wall disruption. However, prolonged heating led to substantial degradation of heat-sensitive compounds, particularly anthocyanins, resulting in pronounced color alterations and a marked decline in antioxidant activity. Individual phenolic analysis demonstrated that rutin remained relatively stable during boiling, whereas quercetin, <i>p</i>-coumaric acid, and vanillic acid increased, while chlorogenic acid, epicatechin, 4-hydroxybenzoic acid, and quinic acid decreased under prolonged thermal treatment. The findings demonstrate that thermal processing conditions should be carefully optimized to achieve a balance between bioactive compound retention and desirable physicochemical properties, providing practical insight to produce clean-label functional syrups.</p>