The western German state had been held by the conservative CDU since 1999.
German Chancellor Olaf Scholz’s centre-left Social Democrats (SPD) secured a significant victory in their first test at the ballot box since last year's election.
The SPD won Sunday's regional election in the western state of Saarland, which had been held by the Christian Democrat bloc (CDU) since 1999.
The result means that Social Democrat Anke Rehlinger will become the new governor of Saarland — one of Germany's smallest states on the country's French border.
The final results showed the SPD winning 43.5% of the vote and the CDU capturing 28.5% support.
The Social Democrats also took 29 seats in the 51-seat state legislature, an absolute majority — something that has become rare in German politics.
“This is the result of hard work over recent years,” Rehlinger told supporters. “We have won back people’s confidence.”
Scholz’s two partners in the national government, the environmentalist Greens and the pro-business Free Democrats, both narrowly failed to win the 5% support needed to re-enter the legislature in Saarland. Neither party is traditionally strong in the region.
The mandate of the three-party coalition has so far been dominated by Russia's war in Ukraine, which has prompted Germany to upend its defence policy and welcome large numbers of refugees.
The country is also grappling with a persistent wave of coronavirus infections, recently seeing over 200,000 new cases on many days.
The election in Saarland was the first of three state elections within two months — all in regions dominated by CDU. The most important vote on May 15 is in Germany’s most populous state, North Rhine-Westphalia.