"Deadly election-related and communal violence in northern Nigeria following the April 2011 presidential voting left more than 800 people dead... The victims were killed in three days of rioting in 12 northern states," the New York-based rights watchdog said in a statement.
Nigerian authorities have so far refused to give a toll of the violence sparked by the victory of incumbent Goodluck Jonathan, ahead of opposition politician Muhammadu Buhari.
A local rights group had estimated that more than 500 were killed.
The April 16 election won by Jonathan, a southern Christian, led to an explosion of rioting across the mainly Muslim north, the home region ex-military ruler Buhari.
HRW has urged Nigeria to probe the killings and prosecute perpetrators of the violence which also saw churches, mosques and houses set ablaze.
Jonathan last week inaugurated a 22-member panel to investigate the unrest as well as pre-election violence. It is headed by a prominent Islamic scholar.
While the April vote was judged both at home and abroad as "among the fairest in Nigeria's history, but they also were among the bloodiest," said Corinne Dufka, A HRW senior West Africa researcher.
"The newly elected authorities should quickly build on the democratic gains from the elections by bringing to justice those who orchestrated these horrific crimes and addressing the root causes of the violence," she said.
Nigeria, Africa's most populous nation, has a mostly Muslim north and a predominately Christian south.