Wearing black trousers and a red blazer, Merkel smiled for photographers as she dropped her voting paper into a ballot box at a polling station near her flat in central Berlin.
Her husband Joachim Sauer accompanied the chancellor of 12 years into the station, holding up an umbrella to shield them from light rain.
Merkel's conservative CDU party, along with its Bavarian sister party the CSU, holds a double-digit lead in the polls over their nearest rivals, the Social Democrats led by Martin Schulz.
Another term in office for Merkel, who came to power in 2005, could see her match the 16-year reign of her former mentor Helmut Kohl.
Her main challenger, former European Parliament chief Schulz, cast his vote in his western hometown of Wuerselen, telling reporters he was "optimistic" despite surveys suggesting his party was headed for a drubbing.