Red‐fleshed dragon fruit is a promising source of antioxidants and natural colour compounds. This study examined the effects of ohmic heating (OH) and ascorbic acid addition on total phenolic content (TPC), betacyanin (BC), antioxidant activity and colour changes in the juice during storage. The juice was treated from 20 to 85 °C and stabilised at 85 °C for 60 s by OH at 60 Hz and 30 V cm⁻¹. The treated samples were stored at 4 °C in dark bottles for 8 weeks, and BC, TPC, antioxidant activity and colour were determined weekly. After 1 week of storage, there was no degradation of TPC or BC (P > 0.05). However, after 8 weeks of storage, BC decreased by 47%, TPC decreased by 60% and antioxidant activity decreased by 39% in the OH juice. The colour changes of the juice during the 8‐week storage period were strongly correlated with the BC (r (correlation coefficient) > 0.98). A comparison of the OH process with conventional heating (CH) showed that OH resulted in less degradation to BC, TPC and antioxidant activity than CH. OH is a suitable technology for processing red‐fleshed dragon fruit juice. Adding 0.25% AA to the juice helped reduce changes in colour, BC, TPC and antioxidant activity (P < 0.05).