The plant is worth an estimated $367 million and is capable of producing 46 megawatts.
The new plant is expected to prevent about 89,383 tons of carbon dioxide emissions and to power a large portion of the region's homes.
The plant is not far from the border with Spain and has 2,520 Acciona tracker solar panels.
The first 3 megawatts were installed at the end of 2007 and connected provisionally in 2008.
The Amareleja plant will help achieve goals set by the Portuguese government regarding energy efficiency.
The goal is to have 30 percent of Portugal's energy come from renewable sources.