December is mild and damp in Rabat, the Atlantic holding the capital well above the chill of the inland cities. Daytime highs around 18°C are pleasant for walking the Oudayas, the Hassan Tower esplanade and the medina between showers, while evenings turn cool and a warm layer is needed. It is among the wetter months, with grey spells off the ocean, but bright, sharp afternoons frequently break through, and the city is calm and good value.
This is a month for the sheltered and the cultural. The Mohammed VI modern-art museum, the Chellah ruins under low winter light, and the blue lanes of the Kasbah des Oudayas all reward an unhurried visit, and the lighter crowds make it easy to take your time. Plan around the weather rather than against it — a wet morning often clears to a bright, blustery afternoon ideal for the long Bouregreg riverfront.
Outside any festive-season demand, December is a peaceful, low-cost time to enjoy Morocco's elegant capital, and its mild maritime winter is comfortable by Moroccan standards. It pairs easily with Casablanca down the coast and the imperial inland of Meknes and Volubilis, making a relaxed cultural circuit. For a mild-weather city break with rich monuments and few crowds, December is a quietly rewarding choice.