The destination will switch to the new process – which is expected to "substantially improve" processing times for arriving passengers – on 1 March.
All travellers arriving by air have been encouraged to complete the form online prior to arrival to avoid delays.
Once they have landed, travellers will be required to show their receipt to immigration and customs officers.
In the first instance, passengers who have not completed the form will be required to complete and submit the paper form on arrival, but this arrangement will be phased out within six weeks of the electronic form going into operation.
The UK Foreign Office has updated its travel advice to Saint Lucia to reflect the new process.
It said: "All travellers arriving from 1 March 2023 should fill in the online immigration form within 72 hours before arrival.
"The form and information on the process can be found on the St Lucia Tourism Board website. This replaces the written form and should speed up the arrival process.
"Anyone who does not travel with proof of the online form will be able to complete a paper form on arrival until mid-April, as they are being phased out."
