Journalists have been barred from attending the court hearings and Suu Kyi's lawyers have been banned from speaking to the media.
The United States slammed the latest sentencing as an "affront to justice and the rule of law".
"We call on the regime to immediately release Aung San Suu Kyi and all those unjustly detained, including other democratically elected officials," a State Department spokesperson said.
The coup sparked widespread protests and unrest, and renewed fighting with established ethnic rebel groups.
Dozens of "People's Defence Forces" have also sprung up to fight the junta and have surprised the military with their effectiveness, analysts say.
According to a local monitoring group, the crackdown has left more than 2,000 civilians dead and seen some 17,000 arrested.
'Erase the past'
Suu Kyi has been the face of Myanmar's democratic hopes for more than 30 years, but her earlier 11-year sentence already meant she was likely to miss elections the junta says it plans to hold by next year.
"Immune from domestic and international outrage, the punishment trials against Suu Kyi and her supporters are designed to erase the democratic past," independent Myanmar analyst David Mathieson told AFP.
"Their intent is clear to everyone it seems, everyone but the international community."
In June, Suu Kyi was transferred from house arrest to a prison in the capital Naypyidaw, where her trial continues in a courthouse inside the prison compound.
She remains confined to the jail, with her link to the outside world limited to brief pre-trial meetings with lawyers.
Many of her political allies have also been arrested since the coup, with one chief minister sentenced to 75 years in jail.
Last month, the junta stoked renewed international condemnation when it executed Phyo Zeya Thaw, a former lawmaker from her National League for Democracy (NLD) party, for offences under anti-terrorism laws.
Suu Kyi learned of the execution at a pre-trial hearing, a source with knowledge of the matter said, but has yet to speak on the matter.
The junta says it seized power following massive fraud during 2020 elections in which the NLD trounced a military-backed party and which international observers said were largely free and fair.