Government forces trying to recapture the area from so-called Islamic State (IS) militants have cut supply routes, and IS stops people from leaving.
What little food available is being sold at exorbitant prices, forcing some to eat soup made from grass, HRW says.
Tens of thousands of residents remain in the city, which is one of two remaining IS strongholds in Iraq.
"The people... are besieged by the government, trapped by Isis [IS], and are starving," said Joe Stork, deputy Middle East director. "The warring parties should make sure that aid reaches the civilian population."
HRW said Iraqi activists who were in contact with Falluja residents reported that people in the city were reduced to eating ground date seed flat bread as well as grass soup.
And the ingredients for such meagre meals are highly priced. A 50-kg sack of flour is sold for US$750 and a bag of sugar for $500, whereas the same amount of the same ingredients in Baghdad, about 50km (30 miles) east, cost $15 and $40 respectively, the HRW cited one Falluja resident as saying.
According to a medical source, starving children arrive at a local hospital every day.
Any attempt to escape from Falluja is extremely dangerous, says Alan Johnston, BBC World Service Middle East editor. There are reports of people being executed by IS for trying to leave.
Iraqi security forces backed by the US-led coalition are stepping up the fight in and around the towns and cities still held by IS.
But it seems that the militants confronting the surrounding government forces are determined to keep civilians with them in Falluja, perhaps to act as human shields, our correspondent says.
Residents in the Iraqi city of Falluja are starving, says US-based campaign group Human Rights Watch (HRW).