Aims Current guidelines recommend that patients with ST-elevation myocardial infarction (STEMI) receive primary percutaneous coronary intervention (PCI) with a door-to-balloon (D2B) time of 90 minutes. This time frame has been difficult to achieve in developing countries. We aimed to test the hypothesis that using WhatsApp for inter-hospital transfer in STEMI primary PCI would result in shorter D2B time through avoiding emergency department (ED) waiting. Materials and methods In the period from May 2018 to April 2019, 300 patients with STEMI treated with primary PCI were prospectively enrolled in a multi-center observational study covering 3 PCI-capable centers in Cairo city, Egypt. Patients were categorized into 3 groups: G-1; Self-presenting to the ED, G-2; Transferred by ambulance to the ED and G-3; those transferred from non-PCI capable centers directly to catheterization laboratory (bypassing the ED as possible) after the team was pre-activated through an ECG image sent using WhatsApp. The primary endpoint was between-groups difference in total ischemic and D2B times. Results All groups were matched regarding baseline criteria. WhatsApp use was associated with significantly shorter D2B time (166.5±33.8 vs 157.8±29.6 vs 120.8±35.2 minutes in groups 1, 2 and 3 respectively, P < 0.001) and shorter total ischemic time (258.9±33.9 vs 249.6±29.4 vs 212.7±35.4 minutes in groups 1, 2 and 3 respectively, P < 0.001). Linear regression showed that WhatsApp referral significantly reduces ED waiting time by about 20.4 minutes (95% CI -21.6 – -19.3 minutes, P < 0.001). Conclusion Using WhatsApp for inter-hospital transfer in primary PCI is associated with significantly shorter D2B times.