Some 300 Marines will form part of NATO's train, assist and advise mission in the province where the Taliban control or contest 10 of the 14 districts.
US Marines on Saturday returned to Afghanistan's volatile Helmand province, the first to be deployed in the war-torn country since NATO forces ended their combat role in 2014.
Commander of US and NATO forces in Afghanistan General John Nicholson attended a handover ceremony marking the return of the force in the poppy-growing southern province, where American forces faced heated fighting until 2014.
Some 300 Marines will form part of NATO's train, assist and advise mission in the province.
The ceremony marking the deployment, first announced in January, came one day after the resurgent Taliban announced the launch of their "spring offensive", heralding a fresh fighting season.
The US has around 8,400 troops in the country with about another 5,000 from NATO allies.
Their fresh deployment in the province is the latest sign of how foreign forces are increasingly being drawn back into fighting the insurgency.
The Taliban control or contest 10 of the 14 districts in Helmand.
It is the deadliest province for British and US troops over the last decade and blighted by a huge opium harvest that helps fund the insurgency.
The Marines were among the first US forces sent to Afghanistan after the 2001 terror attacks in the US.