B'Tselem said there were 320 minors under the age of 18 among the civilian dead.
The Israeli military has said 1,166 Palestinians were killed during the offensive it dubbed Operation Cast Lead - 709 combatants, 295 civilians and 162 people whose status it was unable to clarify.
Colonel Avital Leibovich, an Israeli Army spokesperson, dismissed the new figures.
"B'Tselem sources as specified in its own report are based on blogs, and Palestinian websites, some of which belong to the military wing of Hamas. Now, these sources are not reliable sources.
"The process by which the intelligence in the Israeli Defence Forces has conducted the investigation was a very thorough one. It took over two months to complete this process ... we can clearly say that the majority of the casualties, 709, were Hamas," she told journalists.
But in a statement, B'Tselem said its researchers had visited Gaza homes to take witness testimony and collect documents to arrive at its casualty figures. It called on the Israeli government to initiate an independent investigation of the war.
"The extremely heavy civilian casualties and the massive damage to civilian property require serious introspection on the part of Israeli society," B'Tselem said.
The Israeli military has said it operated during the war in accordance with international law and had tried, as much as possible, to minimise civilian casualties.
In its mission statement, B'Tselem says it endeavours to document and educate the Israeli public and policymakers about human rights violations in Israeli-occupied Palestinian territory.