Using the Yen Sign
The yen sign is a capital Y with one or two horizontal strokes, and it does double duty: Japan uses it for the yen and mainland China uses it for the renminbi yuan. Both currencies happen to share a name that descends from the same character for "round", so the sign followed. Unicode has one code point, U+00A5, and leaves the choice of one or two crossbars to the font, which is why Japanese typefaces usually show two bars while mainland Chinese typefaces often show one.
Before the number, never after
In both countries the sign goes before the amount with no space, ¥3,500. Japanese amounts are whole numbers because the sen sub-unit is out of circulation, while yuan prices can carry two decimals for jiao and fen. Japanese text written in kanji reverses the order and drops the sign altogether: 3,500円. Chinese does the same with 元, so 100元 and ¥100 mean the same thing in a Shanghai shop. When you quote figures for readers who could be thinking of either currency, put the ISO code in front instead: JPY 3,500 versus CNY 100. Financial pages also write JP¥ and CN¥ when they want to keep the sign.
The fullwidth twin
Typing on a Japanese or Chinese keyboard usually produces ￥ at U+FFE5, the fullwidth yen sign, which occupies the same square cell as a CJK character. It exists so that rows of mixed text line up in monospaced layouts. In a bio or a message it is harmless, but in a spreadsheet or a price database the two characters are not equal, so a search for ¥ will miss every ￥ row. Pick one and stay with it; the halfwidth ¥ is the safer default because Western fonts always include it.
The backslash problem
Japanese computers historically used a code page where the byte for backslash was drawn as ¥. That is why a Windows file path on a Japanese PC appears as C:¥Users¥name, and why some old web pages show ¥ where the author typed a backslash. Modern UTF-8 text keeps them separate, but if you paste from a legacy Japanese document, check that the currency sign is really U+00A5 and not a backslash the font is disguising.
Keyboards and rendering
Because ¥ belongs to Latin-1, it is on every device: Alt + 0165 on Windows, Option + Y on a Mac, and a long press on the dollar key on phones. Japanese hardware keyboards have a dedicated ¥ key at the top right. The sign renders in usernames, game chats and any font that covers basic accents, which makes it more dependable than the newer rupee sign and won sign.
In social bios the sign marks Japan-based or China-based creators, commission prices in yen, or simply an anime aesthetic, often surrounded by characters from the aesthetic symbols collection.