Six people were injured, among them four police officers, in the attack on the Tree of Life Congregation in Squirrel Hill. The officers are in a stable condition.
The suspect has been named as Robert Bowers, 46. He has been described as a heavy-set, bearded white man.
Here’s what we know so far about the shooting.
Warning: This story contains offensive language and anti-Semitic quotes.
Emergency services arrived at the scene at about 10:00 local time (14:00 GMT), during a service.
Two officers – among the first responders – were injured in an initial confrontation with the gunman.
A further two Swat team members were hurt inside the building, clashing with the shooter.
All four are in a stable condition, authorities said.
A 61-year-old woman and a 70-year-old man are also currently being treated. The man had gunshot wounds to the torso and is in a critical condition.
The gunman is now in custody after surrendering to the authorities.
The crime scene was “horrific”, Pittsburgh’s Public Safety Director Wendell Hissrich told reporters. “One of the worst I’ve seen, and I’ve [worked] on some plane crashes. It’s very bad,” he added.
Who are the victims?
Details about the 11 people killed are starting to emerge.
Officials confirmed on Saturday that no children were among the casualties.
According to a criminal complaint filed against the accused gunman, eight men and three women were killed in the attack.
The victims’ identities are expected to be released by officials at 09:00 local time on Sunday (13:00 GMT).
His nephew, Steven Halle, described him as “somebody that everybody liked” and a “kind soul”.
He said his uncle had recently become a grandfather and attended the synagogue every Saturday for services.
What is the synagogue?
The Tree of Life was set up more than 150 years ago, its website says.
It merged with nearby Or L’Simcha congregation about five years ago, to form the Tree of Life – Or L’Simcha Congregation.
The synagogue is in Pittsburgh’s east-end Squirrel Hill area, which has a large Jewish community dating back to the 1920s.
The gunman entered the building during a baby naming ceremony.
The Jewish Federation of Greater Pittsburgh’s president Jeff Finkelstein told reporters up to 50 or 60 people regularly worshipped there on Saturday morning.
According to the calendar on its website, Shabbat morning services were scheduled between 09:45 and 12:00.
A Shabbat training programme for children “in kindergarten through 7th grade” was set to run from 10:15 to 11:45, at which attendees learn prayers, take part in the service and study the Torah.
Extra police officers have been deployed at synagogues and Jewish centres across the US after the attack.
What do we know about the gunman?
The suspect has been named as Robert Bowers.
FBI special agent Bob Jones said that Mr Bowers did not appear to be known to authorities prior to events on Saturday.