The change came into force at midnight last night and is in addition to the other Bailiwick-wide lockdown restriction.
Travellers arriving in Alderney on essential work, such as medical and care professionals, will be exempt from the rule.
There will also be exemptions on compassionate grounds, such as for those returning from medical treatment.
Travellers from Alderney to Guernsey do not need to self-isolate. No cases of Covid-19 have been detected in the northern island.
Civil servants are working to set up a suitable system for exemptions.
This new regulation only applied today, but people who travelled to Alderney recently are being urged to voluntarily self-isolate or reduce contact with others.