The blast ripped through a crowded bazaar in the town of Lashkar Gah.
A spokesman for the provincial governor told the Associated Press news agency that the bomber had blown himself up opposite a police station.
Helmand province has seen some of the worst of the violence that has killed hundreds in Afghanistan this year.
At least 47 people were wounded in the latest blast, six of them critically, Hanif Khan, a local hospital official, told the Associated Press news agency.
Shattered glass and blood-soaked turbans were scattered at the site of the bomb, AP quoted security guard Hayatullah Khan as saying.
Afghanistan is going through its bloodiest period since the fall of the Taleban in 2001.
Militants recently stepped up their insurgency against government and foreign forces in the south and east.
About 2,000 people, most of them militants, but also civilians, aid workers, Afghan forces and more than 90 foreign soldiers have been killed this year.
Last week, US-led forces killed seven men and a child in a raid in Kunar province in eastern Afghanistan.
The coalition forces said the men were suspected al-Qaeda members, but local people told the BBC that the men were tribal elders who had gathered to resolve a local dispute.
Source:BBCLast Mod: 00 0000, 00:00