Ploudalmézeau (; ) is a commune in the Finistère department of Brittany in north-western France.
The village and port of Portsall is part of the commune. The Amoco Cadiz oil tanker ran aground off Portsall in 1978, causing a large crude oil spill.
Ploudalmézeau has an oceanic climate (Köppen climate classification Cfb). The average annual temperature in Ploudalmézeau is . The average annual rainfall is with November as the wettest month. The temperatures are highest on average in August, at around , and lowest in January, at around . The highest temperature ever recorded in Ploudalmézeau was on 18 July 2022; the coldest temperature ever recorded was on 28 February 2018.
Ploudalmézeau is twinned with Cullompton, Devon.
In 2008, 11.69% of primary-school children attended bilingual schools, where Breton language is taught alongside French.
On 16 March 1978, Amoco Cadiz, a very large crude carrier (VLCC), owned by Amoco, split in three after running aground on Portsall Rocks, 5 km (3.1 mi) from the coast of Portsall, resulting in the largest oil spill of its kind in history to that date.