Map from Google - Brittany

The climate of Brittany, region of north-western France, is oceanic, with relatively cold winters and cool summers, and with frequent cloud cover and wind throughout the year, however, the weather in summer is more forgiving.
Here are the average temperatures in Saint-Malo, located in the northern part of the coast.
Average temperatures - Saint-Malo
Min (°C)33469111313121064
Max (°C)891113172022222016129

Precipitation is abundant on the west coast (see Brest), where it exceeds 1,100 millimetres per year, while it goes down to 950 mm on the south coast (see Lorient), below 800 mm along the north coast (see Saint-Malo), and about 700 mm in the eastern inland area (see Rennes), but the rains are frequent throughout the year everywhere, although the least rainy season is summer, when it rains for about 10/12 days per month.
Here is the average precipitation.
Average precipitation - Saint-Malo
Prec. (mm)807065505550405060759590780

Winter, from December to February, is quite cold: the temperatures are usually above freezing, but wind and humidity increase the feeling of cold. Besides, very cold periods may occur, with temperatures below freezing, when France is affected by currents coming from Russia. In these cases, there can be snow, generally not abundant, and frost; the coldest records are around -10 °C. On the contrary, when the wind blows more from the ocean, on milder days the temperature can reach 12/14 °C even in winter, although the wind can be strong.

Spring, from March to May, is very cool, if not cold, especially in the first part. In March it can still snow; in April, and sometimes in the first half of May, there can still be cold days. The rains are frequent, but less than in winter. Besides, the days get longer, and you can even see the sun, maybe between a shower and the other, or with clouds running in the sky.

Summer, from June to August, is generally cool; some days can be pleasantly warm, but refreshed by lively afternoon breezes. The coolness is felt especially along the west coast: in Brest highs July and August are around 20 °C; on the north and south coasts, in cities like Saint-Malo and Lorient, they are around 21/22 °C, while in Rennes, situated at a certain distance from the sea, they reach even 23/24 °C. However, the temperature varies according to weather situation: when Atlantic disturbances pass (which may occur even in summer), the weather is cool and cloudy, with some chance of rain, but when the Azores anticyclone succeeds in reaching, maybe for a few days, this north-western part of France, there can be nice and sunny days. Sometimes it can even be hot: occasionally the temperature can touch 30 °C on the coast, and even 35 °C in Rennes.

Autumn, from September to November, is still mild and pleasant in September, but later it gradually becomes cold, dull and rainy, as well as windy.

The amount of sunshine in Brittany is low in late autumn and winter, while it becomes quite acceptable from April to September. Of course, we are still in the least sunny region of France (along with Normandy and the north-east), but you can hope for some fine day, or more easily for a variable weather.

The temperature of the sea in Brittany is very cool even in summer, and does not invite tourists to swim, at least those who suffer from the cold: it reaches 17 °C in August in Brest, and 18 °C in Saint Malo. Here are the average sea temperatures.
Sea temperature - Saint-Malo
Sea (°C)1099101214171817151312

Saint Malo

When to go

The best time to visit Brittany is summer, from June to August, being the warmest of the year, although even in this season it's better to bring a sweatshirt, a light jacket and umbrella. June is a bit cooler, therefore you may prefer July and August. September is also a good month: it's a bit rainier than the summer months, but the temperatures are still pleasant, especially in the first part.
In Saint-Malo, frequented mainly by tourists from central and northern Europe, as we have seen the sea is very cool even in summer, however, on sunny days, when the breeze is not too cool, you can try to sunbathe.