Éponine Thénardier (/ˌɛpəˈniːn təˈnɑːrdieɪ/ EP-ə-NEEN tə-NAR-dee-ay, French: [epɔnin tenaʁdje]), also referred to as "Ponine", the "Jondrette girl" and the "young working-man", is a fictional character in the 1862 novel Les Misérables by Victor Hugo.

The character is introduced as a spoiled and pampered child, but appears later in the novel as a ragged and impoverished teenager who speaks in the argot of the Parisian streets, while retaining vestiges of her former charm and innocence.

Éponine is born in 1816, the oldest child of the Thénardiers. As children, Éponine and her younger sister Azelma are described as pretty, well-dressed, and charming. They are pampered and spoiled by their parents, the Thénardiers, who run an inn in Montfermeil, France. Three years later, when Fantine and her illegitimate daughter Cosette come across the inn, Fantine sees Éponine and Azelma playing outside. Cosette joins the two sisters and the three play together. Fantine asks the Thénardiers to take care of Cosette while she goes to look for work in her hometown. The Thénardiers abuse Cosette, dress her in rags and force her to work, while spoiling their daughters and letting them play. Following their parents' example, Éponine and Azelma are unkind to Cosette and treat her like a servant.

During the 1823 Christmas fair, Éponine and Azelma admire a big, beautiful and expensive doll in a shop window. That night, they play with their own doll, while ignoring Cosette. Their doll is "very much faded, and very old and broken". They then cast the doll aside to play with a kitten. Cosette, never having owned or played with a doll, quickly grabs it and attempts not to be noticed with it. After fifteen minutes, Éponine and Azelma discover that Cosette has taken their doll and tell their mother, who yells at Cosette. Jean Valjean, who has witnessed the scene, leaves the inn and buys the expensive doll. He returns a moment later and gives it to Cosette. For the first time, Éponine and Azelma "looked upon Cosette with envy".

Hugo depicts Éponine again in 1832. After her family's bankruptcy of the inn, they move to Paris and live in a small run-down apartment at Gorbeau House under the assumed name of "Jondrette". They live next door to the apartment of Marius Pontmercy. In adolescence, Éponine becomes a "pale, puny, meagre creature", with a hoarse voice like "a drunken galley slave's", having been "roughened by brandy and by liquors". She wears dirty and tattered clothing, consisting solely of a chemise and a skirt. She is missing a few teeth, is barefoot, has tangled hair, bony shoulders, and heavy brooding drooping eyes, while the "grace of her youth was still struggling against the hideous old age brought on by debauchery and poverty" and has only a trace of beauty lingering upon her face. She had "the form of an unripe young girl and the look of a corrupted old woman; fifty years joined with fifteen; one of those beings who are both feeble and horrible at once, and who make those shudder whom they do not make weep". She and Azelma help their father, who writes to rich people under different names, to beg for money. The two sisters pass Marius while running away from the police, unaware that they dropped their package of forged letters begging for alms. Marius takes the package back to his apartment.

The next day, Éponine visits Marius at his apartment and gives him a letter from her father begging for money. Marius notes that the handwriting and the stationery are identical to the letters in the package. Éponine looks around Marius' room and goes to look at Marius' mirror while singing to herself. To impress him, she proves that she is literate by reading aloud a passage from one of his books and writing "The cognes (police) are here" on a piece of paper. Éponine tells Marius that she sometimes goes to the theatre, that he is handsome, and mentions that she has noticed him a number of times before. Changing the subject, Marius hands her back the other letters. Éponine is grateful to him for finding them. She then tells Marius that she sometimes walks by herself at night, how she and her family lived under the arches of bridges the previous winter, how she contemplated drowning herself and that she had hallucinations due to lack of nourishment. Pitying her, Marius gives her five francs, and she thanks him in a chain of argot. Éponine leaves, but not before picking up a piece of hardened bread from Marius' bureau and biting into it.

After Éponine leaves, Marius observes her and her family in their apartment next door. They live in utter squalor. Marius hears Éponine claim that she has arranged for a philanthropist from the local church to come to their home and give them money to help pay the rent. In an effort to make his family look poorer, Thénardier orders Azelma to punch out the window, which she does, cutting her hand open. It turns out that the "philanthropist" is in reality Jean Valjean, as yet unrecognized by the Thénardiers and known by a different name, and he visits to inspect their circumstances. He is accompanied by Cosette, with whom Marius is in love. Valjean promises to return later with money.

Marius attempts to follow Valjean and Cosette but is unable to pay for a cab, having given Éponine his remaining five francs. When he returns to his room, Éponine asks what is troubling him and offers to assist him. Marius asks her to find the address of the father and daughter that just visited. Éponine reacts bitterly, realizing his romantic interest in the philanthropist's daughter, but agrees to do so after he promises to give her anything she wishes in return.