The
River Meu forms most of the commune's northern border.
Climate Merdrignac has an
oceanic climate (
Köppen climate classification Cfb). The average annual temperature in Merdrignac is . The average annual rainfall is with December as the wettest month. The temperatures are highest on average in July, at around , and lowest in December, at around . The highest temperature ever recorded in Merdrignac was on 5 August 2003; the coldest temperature ever recorded was on 12 January 2010. {{Weather box ==Population==