The National Remembrance Service was held at Christchurch Arena, where the names of the victims were read aloud while their pictures were displayed on a large screen. A minute of silence was also observed for the victims.
Community members sang songs and delivered prayers during the memorial, while Ardern spoke about how "words have the power to heal".
The attack carried out by a lone gunman took place across two mosques in the city of Christchurch and was targeted at Muslims attending Friday prayers.
New Zealand's worst peacetime killing shook the country to its core and prompted the government to quickly tighten gun laws.