Paris in December: 8°C afternoons, 4°C mornings, ~11 rain days, 8h 10m daylight, sunset at 4:55pm.
December is Paris at its most deliberate. The city dresses for the walk between lit windows — Galeries Lafayette's Christmas vitrines, the Place Vendôme lights, the Louvre pyramid in the dark at 5pm. Météo-France records 8°C afternoons against 4°C mornings, ~11 rain days, and just 8h 10m of daylight (sunset at 4:55pm at mid-month). The dressing answer is layered wool and proper outerwear. A long below-knee wool coat in black or charcoal does the heavy lifting; underneath, a cashmere turtleneck, a chunky knit, lined wool trousers, and knee-high leather boots. Gloves are daily; a hat starts appearing by mid-month; the scarf is already doing double work. Evenings mean a wool skirt with opaque tights and the same boots and coat — the outfit stays constant; only the bottom changes.
December is Paris at its most deliberate — the city dresses for the walk between lit windows.

The December anchor. Below-knee, double-breasted, wool or wool-cashmere. Collar that can flip up against the Seine wind. Max Mara, Toteme, Harris Wharf London, or a vintage shop around rue de Rivoli if you hunt.

The December base layer. Fine-gauge cashmere under a chunky wool knit is the warmth secret no one with a December Paris trip packs by accident. Uniqlo for the accessible version; &Daughter or The Elder Statesman for the splurge.

Same knit from October and November, doing its third month. The collar-over-turtleneck look is the most-repeated Parisian December formula. If packing one sweater for Paris in December, make it this one.

Half-lined or fully lined, centre crease, in dark navy or charcoal. Handles the 4°C morning and the 8°C afternoon without the shock of cold denim. Tucks into boots for the coldest days.

The evening bottom. Any wool or wool-blend midi in a dark solid; worn with 80-denier opaque tights and the knee-high boots. The formula Jeanne Damas wears to openings every December in Paris.

Taller than November's mid-calf. Slouchy or fitted, dark brown or black, low block heel. Knee-high means the cold does not find the calf seam. Waterproof-treat them the week before the trip.

Thicker than November's. Big enough to double-wrap the neck and still cover the chin. Navy, charcoal, burgundy, or camel — solid, not pattern. Acne Studios oversized is the Parisian default.
Cashmere turtleneck under the chunky knit · navy wool trousers · knee-high boots · heavy scarf · long wool coat buttoned · gloves · beret · crossbody. Morning espresso at Terres de Café in the 3rd, walk through the Marais in the last of the morning light.
Turtleneck tucked into wool skirt · opaque tights · boots · scarf · coat · gloves · beret · crossbody. 6pm ice-rink at Grand Palais, then mulled wine at Marché de Noël Tuileries; the sun set an hour before you left the hotel.
A suggested look — Navy long-sleeve button-down shirt with band collar, Dark brown fitted mini skirt in chocolate tone, Knee-high slouchy grey-brown leather boots with stacked heel, Slouchy tan leather shoulder bag with gold hardware and charm details.

Cool-cold, wet, and short on daylight. Météo-France records an average high of 8°C and an average low of 4°C, around 11 rain days, and just 8h 10m of daylight at mid-month (sunset at 4:55pm). Expect 2–3 days in the 1–3°C range. The wind off the Seine makes it feel colder than the numbers suggest.
A long below-knee wool or wool-cashmere overcoat in black, charcoal, or navy, double-breasted. Puffer coats and parkas read wrong on Paris streets; a proper wool coat handles 4°C mornings with a chunky knit underneath, and it looks right in every arrondissement. Bring a hat and gloves as well — this is the month they become daily.
Yes, for specific reasons. The Christmas windows at Galeries Lafayette and Printemps Haussmann are the best in Europe. The Marchés de Noël along the Tuileries and around Notre-Dame run through Christmas Eve. Museums are less crowded. The tradeoff is the weather — pack for 4°C with wind and 8h of daylight and it is genuinely lovely.
A long black or charcoal wool coat over a fine-gauge turtleneck, a chunky knit, lined wool trousers or a wool midi skirt with opaque tights, and tall leather boots. Gloves daily, a beret by mid-month, a heavy wool scarf worn every day. The palette is dark: black, charcoal, navy, camel, burgundy. Nothing bright, nothing logo-heavy.
Very short. Sunrise is around 8:30am, sunset around 4:55pm at mid-month — 8h 10m of daylight. The practical impact is that outdoor sightseeing needs to happen before 3pm; museums, department stores, and restaurants carry the late afternoon. Schedule the Eiffel Tower or a Seine walk for the blue hour at 5pm when the lights come on.