Côtier vs. littoral
While both côtier and littoral translate to "coastal" in English, they serve different functions in French. Côtier is generally used for physical proximity to the shore and maritime activities close to land, whereas littoral is a broader term often used in scientific, administrative, climatic, or ecological contexts referring to the coastal region.
Côtier
B1Derived from the word côte (coast/rib), this adjective usually refers to things that are physically adjacent to the sea or activities associated with waters very close to the land. In nautical terms, it is the opposite of hauturier (deep-sea or offshore).
La pêche côtière se pratique sur de petits bateaux.
(Coastal fishing is done on small boats.)
Nous avons suivi le sentier côtier pendant trois heures.
(We followed the coastal path for three hours.)
Ce navire n'est pas équipé pour quitter les eaux côtières.
(This ship is not equipped to leave coastal waters.)
Le brouillard côtier s'est levé rapidement ce matin.
(The coastal fog lifted quickly this morning.)
Ils ont acheté une petite maison côtière en Bretagne.
(They bought a small coastal house in Brittany.)
Littoral
B2This word relates to the littoral zone, which is the wider interface between the land and the sea. It is often used to describe climates, large geographical zones, ecology, or administrative laws regarding the seaside. It sounds slightly more formal or technical than côtier.
Le climat littoral est souvent plus doux qu'à l'intérieur des terres.
(The coastal climate is often milder than inland.)
Le gouvernement a voté une nouvelle loi pour la protection du domaine littoral.
(The government voted on a new law for the protection of the coastal domain.)
L'urbanisation a modifié le paysage littoral de la région.
(Urbanization has changed the coastal landscape of the region.)
On trouve une faune variée dans la zone littorale.
(A varied fauna is found in the coastal zone.)
Cette route traverse tout le bassin littoral méditerranéen.
(This road crosses the entire Mediterranean coastal basin.)
Summary
To choose correctly, think about the focus of your sentence. If you are talking about boats, navigation, or a physical specific object right on the water's edge, use côtier. If you are discussing the climate, the region as a whole, ecology, or government zoning, use littoral.