Uniqlo's nylon cross-body bag has been a huge hit online
The legal case filed in Tokyo alleges that some Shein bags "closely resemble" Uniqlo's own and could dent customer confidence in the brand.
Uniqlo's cross-body bag has been a huge hit online, with TikTokkers showing off how many items they can fit in it.
Shein did not immediately respond to the BBC's request for comment.
Uniqlo is demanding that Shein stops selling its version of the bag immediately, as well as asking for "compensation for damages incurred".
Uniqlo's owner, Fast Retailing, said on Tuesday that a petition had been filed in the Tokyo District Court against Shein Japan and two subsidiaries in late December.
Videos of the bag have generated millions of "likes" online over the past year, with one jokey post showing a user taking out a laptop, water bottle, candle and even a robot vacuum and toolkit.
It has been likened to the magical carpet bag carried by Mary Poppins who, in the eponymous 1964 musical, pulls a hat stand and lampshade out of her holdall.
The nylon "round mini shoulder bag", costing £14.90 in the UK, has reportedly become the brand's best-selling bag ever.
Shein, founded in 2008 in China, saw sales surge during the pandemic, fuelled by a jump in online shopping and its social media-focussed marketing strategy.
According to its website, the company employs nearly 10,000 people worldwide and sells to more than 150 countries.
It is now based in Singapore and reports have been circulating that it is considering a stock exchange listing in New York.
It has, however, attracted controversy. Low prices have raised questions about the company's environmental footprint and employment practices, but Shein says it produces clothes in small batches, which is more efficient and means little goes to waste, and that it has zero tolerance policy on forced labour.
It has also seen accusations made online by smaller business owners over claims that their products have been copied by the fashion giant, although it has said that it "respects designers and artists, and the intellectual property rights of others".
Fast Retailing, Japan's biggest clothing seller, opened the first Uniqlo store in 1984 and now operates some 2,500 around the world, many of them in China.
Uniqlo has gained a reputation for practical clothes offered up in a number of different colours, with a focus on thermals and coats in particular.
In its latest financial update, Uniqlo's owner reported profits of 146.7 billion yen (£791m) in the three months to the end of November, compared with 117.1bn yen a year before which has been fuelled by overseas sales.
It is currently projecting a record profit for 2024, but bosses recently said that climate change could pose an increasing challenge for sales of items like its fleece jackets.
-- Courtesy of BBC News