Both inhaled fumes from the decaying weed on a beach in Saint-Michel-en-Greve in Brittany.
A necropsy revealed the horse suffered a pulmonary oedema resulting from inhaling the gas.
The rider, 27-year-old Paris veterinarian Vincent Petit, was dragged from the beach unconscious.
Hydrogen sulphide gases created by the composting sea lettuce can build up under the dry surface crust of the washed-up weed.
When ruptured, the gas escapes and can cause death within minutes.
Local media report that several locals have required hospital treatment, including a council worker who fell unconscious while working to clear up the weed.
There are suggestions that rising nitrate levels in seawater, resulting from runoff from intensive farming in the region, has resulted in an explosion of the weed.
An estimated 70,000 cubic metres of seaweed is cleared off Bittany beaches every summer.