Emergency services said the lorry rolled over on the viaduct and fell onto a road below causing the death of two men, one aged 52 and the other 47.
The accident happened just after 9 o’clock in the morning.
The ambulance service said the vehicle had fallen from a height of about 80 metres.
The A9 Lisbon outer circular links the Estoril coast to Alverca via Queluz, Odivelas and Loures. TPN/Lusa