The woman and three children died in Thailand's resort island of Phuket, a ministry official said, adding the government was trying to confirm a report that at least seven Japanese tourists were swept away in Sri Lanka.
The death toll from Sunday's powerful quake off Indonesia and massive tidal waves it unleashed has reached 28,900 in eight Asian countries and two in Africa.
In Japan, an emergency survey of 17 major domestic travel agencies showed at least 22 of their 1,315 customers who went to the effected region were unaccounted for.
But the foreign ministry official said that number could rise.
Tokyo has urged travellers to postpone all but necessary trips to the tsunami-hit areas, saying visitors may face trouble due to damaged hotels, transportation and telecommunications.