The Thai cabinet is preparing to mull a proposal to offer immunity as part of reconciliation efforts to draw a line under years of unrest since the 2006 coup that toppled prime minister Thaksin Shinawatra, brother of the country's current leader.
But a statement from HRW warned that if adopted the move would "undermine justice", particularly for those killed or injured in deadly crackdowns on "Red Shirt" street protests in Bangkok in 2010.
"Those harmed in the upheavals and their families are still waiting for justice because successive governments haven't kept their promises to hold the abusers accountable," said HRW Asia advocacy director John Sifton.
Thailand has seen in a string of violent protests since 2005, involving the arch royalist and nationalist Yellow Shirts, the mainly rural working-class Red Shirts and several smaller factions.
The current government of Yingluck Shinawatra came to power last year, bolstered by support from the Red Shirts, with a promise of reconciliation in the deeply divided nation.
More than 90 people, mostly civilians, were killed and nearly 1,900 wounded during the 2010 Red Shirt rallies, which ended in a bloody military crackdown under the previous premier, Abhisit Vejjajiva.
Thailand in January agreed a 2.0 billion baht ($65 million) pot to compensate all victims of political unrest in recent years, but criminal investigations into the 2010 deaths have yet to lead to any prosecutions.
The opposition has accused the government of trying to use the amnesty plans as a means of enabling a return to Thailand for Thaksin, who lives abroad to avoid a jail term for corruption and terrorism charges relating to the 2010 violence.
HRW said families welcomed the reparations but expressed concern that actions in parliament would "block accountability and keep the truth buried".
"The reconciliation proposal is about enabling powerful people on all sides to get away with grievous crimes. Everyone wins, except the victims," Sifton said.
Amnesty International has also criticised the amnesty plans, urging Thailand not to grant immunity for "grave human rights violations".
A government spokesman told AFP that it was unclear when the proposals would be discussed by the cabinet.