A United Nations expert panel reported in June that Rwanda's support for M23 had declined in recent months, but HRW said the group still received training and supplies and was able to recruit in Rwanda.
"It does appear the support is more limited than it was last year, but what we have documented in terms of support is still quite significant," said HRW researcher and report author Ida Sawyer.
The M23 rebellion was launched in April 2012 by a group of Congolese army mutineers who claimed a March 23, 2009 agreement to integrate them into the army had not been respected by the government.
The group seized control of the strategic city of Goma on the border with Rwanda last November but withdrew in exchange for a promise of peace talks which have repeatedly stalled.
M23 spokesman Kabasha Amani dismissed HRW as a "very partisan" organization on Tuesday.
"It's not a report, these are just rumors," Amani said. "We have grown used to this. It isn't the first time they've said these things."
The report comes amid fresh rounds of fighting between M23 and Congo's army, including clashes 14 miles north of Goma on Monday.
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Associated Press writer Robbie Corey-Boulet contributed to this report from Dakar, Senegal.