Tarragona: Annual Weather Averages. July is the hottest month in Tarragona with an average temperature of 25Â°C (77Â°F) and the coldest is January at 9Â°C (48Â°F) with the most daily sunshine hours at 11 in July. The best month to swim in the sea is in August when the average sea temperature is 26Â°C (79Â°F). Average Temperature: Tarragona.