China has asked Vietnam to investigate reports that a Chinese visitor's passport was handed back with obscenities written on two pages.
Pictures show a four-letter word written on the pages that contain a map including China's "nine-dash line".
The line marks China's claim to territories in the South China Sea.
Those claims, which overlap with Vietnam's, were recently rejected by an international tribunal.
The tourist, surnamed Zhong and from China's Guangdong province was entering Vietnam in Ho Chi Minh City. She told local media that she was "very disappointed at the personal qualities of Vietnamese officials".
The Chinese consulate in Ho Chi Minh City said in a statement the act was "shameless and cowardly", adding that it had "stained the dignity of both China and its nationals".
China has asked Vietnam to investigate the incident and "seriously punish" those responsible for it, the statement said. An investigation is to be conducted, according to Vietnamese officials.