Justice Suleiman Ambrosa, of the election petition tribunal sitting in Abuja, said the court was satisfied with the complaint by the All Progressives Congress (APC) candidate Dakuku Peterside that the April 11 election was fraught with violence, voter intimidation and ballot fraud.
He said the Independent National Electoral Commission of Nigeria should call a fresh election in the disputed oil-rich state within 90 days.
Governor Nyesom Wike, of former president Goodluck Jonathan's Peoples Democratic Party (PDP), had been declared winner of the election, but the APC, which previously controlled the state, described the result as "a rape of democracy".
Tensions were high because former governor Rotimi Amaechi had switched sides from the PDP to the APC in 2013 and became a vocal critic of Jonathan's government.
There were widespread claims from the APC of PDP irregularities in voting in the state at the presidential election on March 28, leading to demonstrations and calls for a re-run.
Rivers voted nearly 95 percent in favour of Jonathan but he lost the election nationally to the APC's Muhammadu Buhari, who secured the first opposition win in Nigeria's history.
