The suspect, a white male, surrendered to police after shooting three officers.
The gunman reportedly walked into the synagogue building and yelled, “All Jews must die,” before opening fire.
The shooting happened during the weekly Shabbat services when the building would have been full of people.
Michael Eisenberg, the past president of the Tree of Life Synagogue, said about 95 people would have been in the building as at the time the shooting occurred.
Local police say they had received several calls from people trapped inside the Synagogue.
Jason Lando, a spokesman for the Pittsburgh police department, said officers were still carrying out searches in the Squirrel Hill neighbourhood.
Reacting to the incident, US President Donald Trump tweeted: “Watching the events unfolding in Pittsburgh, Pennsylvania. Law enforcement on the scene. People in Squirrel Hill area should remain sheltered. Looks like multiple fatalities. Beware of active shooter. God Bless All!” SHARE THIS STORY USING ANY OF THE BUTTON BELOW ⬇ PLACE YOUR TEXT ADVERT BELOW ⬇⬇⬇