That will see around one billion cubic meters of liquefied natural gas being provided annually by a plant Cheniere Energy plans to build near Corpus Christi in Texas.
The majority of the gas will be used to supply ScottishPower customers in the UK and in Iberdrola's Spanish heartlands. That volume of gas is enough to supply around 750,000 homes in the UK.
The deal will start from 2019 and Iberdrola said it will mitigate some of the impact of supply agreements which will run out around that time.
Iberdrola has existing long-term contracts with companies including Sonatrach, Nigeria LNG, ENI of Italy and Statoil of Norway.