The new prime minister, wearing her civil service dress uniform, crossed her thumbs and index fingers to form a heart shape - a popular gesture in K-pop culture.
The incident has drawn criticism from Ruangkrai Leekitwattana, a serial petitioner known for targeting politicians. Ruangkrai has submitted paperwork to the National Anticorruption Commission, questioning whether Shinawatra's action violated constitutional ethics.
He argues that the gesture, captured on video and widely shared on social media, could undermine public faith in the dignity of the premiership.
This controversy comes amid broader political tensions in Thailand.
Ruangkrai is already pursuing the dissolution of the Pheu Thai Party, claiming it is dominated by Shinawatra's father, Thaksin. Meanwhile, Warong Dechgitvigrom, leader of the Thai Pakdee Party, plans to petition the Election Commission to investigate Thaksin's influence over the party.
Shinawatra, 38, took office on Aug. 16 after her predecessor was removed by the Constitutional Court.