The ship's mission is on behalf of the Department for International Development, Save the Children and the International Federation of Red Cross.
The 16,160-tonne ship sailed from the Sea Mounting Centre at Marchwood, near Southampton, on 3 February tasked with delivering bulky items that are not suitable for air freight.
Her cargo includes 5,700 sheets of corrugated iron to build much-needed shelters, 40 vehicles and 15 containers of general stores.
See Related News to read more about RFA Lags Bay being loaded up for her mission to Haiti.
Largs Bay's Commanding Officer, Captain Ian Johnson, said on arrival at Haiti:
"I am glad that we have arrived in Haiti. We have had some very heavy weather along the way so it is good to be in the calmer waters of the Caribbean so we can concentrate on the job in hand.
"The weather today is like a typical day at home, overcast and grey, and we are conscious that the rainy season is upon us.
"My team are raring to go but before we unload any cargo we will conduct a quick recce of the beach at Port-au-Prince in preparation for the main event which starts at first light tomorrow [19 February]. We expect to take just 14 hours to unload all the cargo."
On their journey members of the crew and embarked military forces participated in a sponsored row on board which so far has raised more than £500 for the Haiti charity appeal.
Surgeon Lieutenant Jim Watchorn, one of the organisers of the event, said:
"We wanted to make a personal contribution as well as delivering the aid and it just seemed the right way to spend our spare time on board."
Sailing on board RFA Largs Bay to Haiti was the Culdrose-based Military Aviation Support Force, which provides the expertise and facilities for the operation of aircraft from ships and land bases abroad.
Their presence in Haiti will greatly improve the distribution of the much-needed supplies around the beleaguered island.
RFA Largs Bay will remain in Haiti after supplies have been offloaded to be tasked as necessary by the UN and the World Food Programme to assist in getting further aid into the country.
To find out more about how RFA Largs Bay is doing, visit her daily blog and meet some of the sailors at sea - see Related Links.
Other British military assets that have been involved in the relief efforts in Haiti include C-130J Hercules aircraft from RAF Lyneham.
One, crewed by 30 Squadron, flew from MacDill Air Force Base, Florida, into Port-au-Prince in Haiti on 23 January 2010.
This was the first UK military air asset involved in the response to the 7.0-magnitude earthquake that occurred 15km south west of Port-au-Prince on 12 January.
On board were 16 members of a UK Military Stabilisation Unit, manned by members of all three Services.
The freight weight totalled 11,839kg, consisting of a Land Rover, a generator, fuel, tents, water and ration packs.
This allowed the team to be self-sufficient for 14 days, working alongside UK Aid - a non-governmental organisation that was scrambled in the immediate aftermath of the earthquake.
The Captain, Flight Lieutenant Tony Hick, commented:
"It was important to get the members of the Stabilisation Unit into Port-au-Prince to show the people of Haiti that the UK is behind efforts to aid reconstruction in areas hit by the natural disaster."