At five o'clock on Wednesday evening the tropical disturbance was about 260 kilometres west of Apia.
The service said the system, which was moving southeast, had a low to moderate potential of developing into a cyclone.
Samoa can expect heavy downpours, strong and gusty winds with flying objects and uprooted trees, it said.
Conditions will be foggy and roads slippery. There is also a danger of landslides, and coastal inundation.
The warning follows several days of bad weather that has caused flooding and forced schools to close in both Samoa and American Samoa.
In response, the Samoa government announced schools would remain closed on Thursday and Friday.