The New York-based watchdog said since March it had interviewed more than 20 migrants and asylum seekers, almost all of whom reported suffering "human rights violations at the hands of Tunisian authorities."
Seven were among "up to 1,200 Black Africans expelled or forcibly transferred by Tunisian security forces" to the country's desert border regions with Libya and Algeria this month.
AFP correspondents reported on Sunday that Libyan border guards rescued dozens of migrants who were visibly exhausted and dehydrated, and who said Tunisian authorities had taken them there.
This followed testimony from NGOs and witnesses to AFP a few days after hundreds were expelled from the Tunisian port city of Sfax in early July.
On Sunday, the EU and Tunisia signed a memorandum of understanding for a "strategic and comprehensive partnership" on irregular migration, economic development and renewable energy.
About 130 kilometres (80 miles) from the Italian island of Lampedusa, Tunisia is a gateway for migrants and asylum-seekers attempting dangerous sea voyages to Europe.
European leaders have been concerned by the flow as well as by Tunisia's economic challenges.
HRW said the seven expelled migrants it spoke with reported that "the military and national guard left them in the desert with insufficient food and water. While some were relocated back into Tunisia a week later by Tunisian authorities, others still needed assistance or were unaccounted for."
The expulsions came as racial tensions flared after a Tunisian man was killed on July 3 in a clash between locals and migrants in Sfax.
HRW said most of the abuses it documented came after President Kais Saied in February accused "hordes" of migrants from sub-Saharan Africa of bringing violence, alleging a "criminal plot" to change the country's demographic makeup.
"Documented abuses include beatings, use of excessive force, some cases of torture, arbitrary arrests and detention, collective expulsions, dangerous actions at sea, forced evictions, and theft of money and belongings," HRW said in its report.
Saied last week said that what Tunisia offered migrants "is better than what they find elsewhere." But he added: "We refuse to be a land of transit or a land of settlement."
Under Sunday's agreement, the EU would grant 105 million euros ($118 million) to Tunis in equipment and financing for the voluntary return of 6,000 sub-Saharan Africans.
HRW said EU members should withhold their support for migration and border management under that deal "until a thorough human rights impact assessment is carried out."