The yo-yo is red with five black dots resembling a ladybug. Ladybug usually attaches it around her hips when she's not using it. The length of the string is unlimited, and can be controlled by willpower. The end of the string has a small red hoop to put around the finger.
In "Stalac Mode", as seen in "Frozer" the yo-yo has a different appearance, having black hexagons over it instead of spots.
The yo-yo is extremely versatile. Ladybug uses it as a weapon, both offensively and defensively, to hit enemies with it like a projectile or tie them up with its string. It works as a grappling hook, which Ladybug can swing from and use to travel far distances. When swung in a circle very quickly, it lights up pink and creates a shield able to deflect and withstand strong attacks. In "Stormy Weather", Ladybug uses the yo-yo's shield to protect herself and Cat Noir from a bus launched at them by tearing a hole through it. In "Gigantitan", the string of the yo-yo is even strong enough to cut through the Eiffel Tower. It can even create safety nets used to catch people from falling or even something as big as a police car.
The yo-yo is useful for some of Ladybug's special abilities. Whenever something corrupts or infects an object or animal, like butterflies infected with the Butterfly Miraculous' energy to become akumas, the yo-yo can capture and purify them back to normal. The yo-yo opens up, showing glowing pink energy, and it closes around the infected animal or object. It opens again with them healed or clean, such as akumas getting purified into ordinary butterflies again or Mr. Squid in the fourth issue of Miraculous Adventures being purified from the unknown black substance. The yo-yo can contain things larger than the yo-yo itself, and it can hold more than one object at once, from a stuffed animal to a huge swarm of akumas.
The yo-yo is also used for Lucky Charm: Ladybug throws the yo-yo in the air, and it releases a pink energy that produces an object that Ladybug uses to save the day.
One side of the yo-yo can slide open, revealing a screen. With this, Ladybug can use the yo-yo as a tracker, a map, a communicator to contact Cat Noir, or a cellular phone. Additionally, it can access the internet, as seen in "Simon Says" and "Pixelator".
In and out of "Aqua Mode", the yo-yo can also work as a breathing apparatus for those who can't breathe underwater like Kim in "Syren", Ladybug herself in "Queen Wasp", "Reverser" and Otis in "Mayura (The Heroes Day - Part 2)".
In concept art and test animation in the 2014 licensing video, the yo-yo appears to have more abilities, like being able to split in half to become a staff, generating a wing-like shield, and exploding like a bomb. These abilities have not yet been seen in the show.
In "Copycat", one side of the yo-yo can be seen with inverted colors (black with red spots) at times, like when Ladybug is using it as a communicator. Thomas Astruc said that it was a mistake that couldn't be fixed.
Antibug also uses a yo-yo like Ladybug's, except that the colors of the yo-yo are inverted to black with red spots.
Ladybug's yo-yo can purify multiple akumas at a time, as seen in "Stoneheart" and "Mayura".
In "Animan", Ladybug mentions that the string of the yo-yo is unbreakable.
In "Miraculous Adventures Issue 4", the yo-yo can also purify or clean other objects that aren't affected by an akuma, like Mr. Squishy.
In "Dark Owl", it's revealed the yo-yo also has an ear communicator and camera so the holder can communicate with others.
The yo-yo is the first Miraculous tool introduced needed for a holder to access their superpower.