The five detained employees of Tigrai TV -- a TPLF-linked channel -- were arrested in May and June this year, the CPJ said Wednesday.
It cited a former senior manager at the broadcaster, as well as multiple media reports and a source familiar with the matter.
The journalists are accused "of 'collaboration with the enemy' for their alleged work with the Ethiopian federal government and its ruling Prosperity Party, according to those sources," the CPJ said in a statement.
"Convictions for collaboration with any group designated as an 'enemy' can carry up to life imprisonment or the death penalty," the media watchdog said.
"Journalists operating in Tigray should be allowed to live and work freely, without fear that they will be targeted in politically motivated cases," said CPJ's representative for sub-Saharan Africa, Muthoki Mumo.
Two of the CPJ's sources said they believed the five were being held "for their work for Tigrai TV during the time when it was controlled by federal authorities," it said.
Calls to a TPLF spokesman by AFP were not answered.
The CPJ also urged Ethiopian authorities to end a communications blackout in Tigray, which has been facing dire shortages of food and disruptions to electricity, banking and other essential services.
The Tigray People's Liberation Front (TPLF), which has been locked in a 20-month conflict with Ethiopia's government, regained control of Tigray in June 2021, overthrowing an interim administration established by federal authorities.
The TPLF mounted a shock comeback in June last year, retaking Tigray and then expanding into the neighbouring regions of Afar and Amhara.
Fifhting intensified in the second half of 2021 before reaching a stalemate.