The vessel, which is called Elise, drifted out into the river after one of its mooring lines parted in the early hours of the morning, according to a Littlehampton Harbour spokesman.
It was then secured and the ship is currently ‘safely grounded’, the spokesman confirmed.
The vessel will refloat when the tides rises, at which point it will be taken to a different berth at the harbour.
The river is still open for navigation, as there is sufficient space to navigate between the vessel and the west bank, according to the Harbour Board.
However a brief closure of the river between the Red Footbridge and Littlehampton Marina will be required at high water today.
Photos show the ship in a diagonal position across the river, in scenes reminiscent of the recent situation in the Suez Canal, where the giant ship Ever Given became stranded.
In a statement on Facebook, a Littlehampton Harbour spokesman said: “The 80 metre ship ELISE entered the harbour at 1.15am this morning. She was moored by 1.45am and discharge of her cargo commenced soon after.
“As the tide fell and she settled onto her mud berth, one of her mooring lines parted at 3.30am.
“The vessel’s stern then moved out approximately 30 metres into the river before coming to rest on the remaining mooring lines.
“Harbour tug ERICA was dispatched to ensure all was stable and harbour staff assisted in putting further lines put ashore.
“The vessel is currently safely grounded along her full length (only further out than usual) with sufficient lines ashore.
“There is no evidence of damage to the vessel. There has been no pollution or injuries.
“She will refloat on the rising tide and the intention is to then return her to an alternate berth at high water (around 1pm) to allow discharge to continue.
“The river currently remains open for the navigation of all craft because there is sufficient space to navigate between the vessel and the west bank.
“A brief closure of the river between the Red Footbridge and Littlehampton Marina will be required at high water today (in a similar to a normal shopping arrival or departure).
“Harbour users should as usual listen to VHF 71 for updates.”