In a report titled "Deported to Danger," the group said it had documented 138 cases of Salvadorans who had been killed after deportation, but stressed the number was likely much higher as there are no official figures.
HRW also found more than 70 cases of deported Salvadorans who suffered sexual violence, torture or other harm, or who disappeared.
President Donald Trump has toughened US immigration policy to make it difficult for Central Americans to seek asylum, forcing thousands to wait in Mexico as their cases are decided. The issue promises to be a cornerstone of his reelection campaign.
"As asylum and immigration policies tighten in the United States and dire security problems continue in El Salvador, the US is repeatedly violating its obligations to protect Salvadorans from return to serious risk of harm," HRW wrote.
The report placed blame on Salvadoran gangs for targeting deportees and the Salvadoran government for failing to protect them. It accused the United States of "putting Salvadorans in harm's way in circumstances where it knows or should know that harm is likely."
The US Department of Homeland Security did not immediately respond to a request for comment.
In the report, HRW said it established a connection in many cases between the reasons Salvadorans had fled their country and the ultimate causes of their deaths.
The report cites the case of Camila Diaz Cordova, a 29-year-old transgender person who applied for asylum in the United States in August 2017 to escape death threats and extortion by multi-national gang Barrio 18.
After her deportation in November 2017, she returned to sex work in San Salvador, the capital, where she was kidnapped and beaten to death by the police, according to a close friend of Diaz Cordova and Salvador's attorney general.
"By losing her bid for asylum or refuge in the United States, or anywhere else, the risk she faced was exactly that: being killed," the friend of Diaz Cordova said in an interview.
Many other deported migrants say their lives are in danger back home.
Luis, 41, worked nearly two decades in California until he was deported in 2015 after failing to appear for an immigration hearing. Luis, who did not wish to use his full name due to fear of reprisals, found work as a bus driver on the outskirts of San Salvador but was perceived as an outsider and a threat by local gangs.
Gang members shot at him and a bullet struck the bus once, though Luis escaped unscathed."They said if they saw me again, they would take me out," Luis told Reuters. The threats have subsided since he began making payments to gangs to be able to drive through their territory, he said.