Mietoinen is a former municipality in the Southwest Finland region of Finland. It was consolidated into the neighbouring municipality of Mynämäki at the beginning of 2007, and since then it has formed part of Mynämäki rather than existing as an independent administrative unit.
As a former municipality, Mietoinen retains its identity as a locality within the wider Mynämäki area. Its administrative history reflects a broader pattern of municipal mergers in Finland during the 2000s, in which smaller municipalities were combined with larger neighbours.