Theodore Tabaro, 49, was sentenced by the Stockholm district court for "genocide" after he was convicted of murder, attempted murder, and kidnappings of the Tutsi minority.
He was acquitted of rape due to lack of evidence.
The incidents occurred between April-May 1994 in Rwanda's southwestern sectors of Winteko, Nyakanyinya and Mibirizi, according to the verdict.
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Tabaro participated in an attack against a Nyakanyinya school in which hundreds of people, including women and children, had been ordered to seek refuge.
The court said he, along with other culprits, "threw hand grenades at the building" and "shot and stabbed" civilians.
Eight hundred people, many of them children, were killed or seriously injured.
He also participated a few days later in an attack against a monastery in Mibirizi.
Investigators and prosecutors in the case interviewed dozens of witnesses and survivors in Rwanda, Europe and North America.
At least 36 deaths and seven rape victims were identified during the investigation.
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Sixteen of the 30 survivors and relatives of victims who lodged the case against Tabaro, will be compensated, the court said.
Tabaro was arrested in October 2016 at his home in Orebro, 160 kilometres (100 miles) west of Stockholm. He arrived in Sweden in 1998 and was naturalised in 2006.
In 2014 and 2016, Sweden sentenced two other Rwandan naturalised Swedes, Stanislas Mbanenande and Claver Berinkidi, to life in prison for participating in the Rwandan genocide.
Triggered after the assassination of Rwandan President Hutu Juvenal Habyarimana on 6 April 1994, the genocide claimed 800,000 lives in three months, mainly among the Tutsi minority, according to UN figures.
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