The short answer
Loose, breathable clothes that cover the shoulders and knees are the easy baseline for the medina, the souks and day trips to Atlas villages. Linen trousers or a long skirt with a light top and a scarf works from midday heat to a cool rooftop dinner. You do not need to cover your hair.
In more detail
Covering up is partly courtesy and partly practical: it noticeably reduces unwanted attention in the souks. Body-conscious or skin-baring outfits draw far more looks in the lanes than clothes that simply cover well, so save short shorts and vest tops for the riad terrace, the pool or the beach.
A scarf is the single most useful thing to pack. It covers your shoulders near a mosque, shields you from the sun on a camel ride and keeps the dust off on an Atlas road.
Marrakech is not uniform. In Gueliz and Hivernage many Marrakchi women wear jeans and fitted tops; in the medina you will see the djellaba, a loose hooded robe, and the hijab. You don't need to copy either. Modest and covered meets the expectation.
Good to know
- Flat shoes or trainers: the medina lanes are uneven and heels are a mistake.
- A light layer for evenings and cold air-conditioning.
- Swimwear stays at the pool; cover up for the walk back.
- Ask for a female guide if you'd like company in the souks.