Archaeological surveys prove that the territory of Vaslui was inhabited since Neolithic. From the 14th century onwards, it developed the provincial town of Vaslui, with a population that varied strongly in the following centuries. The name of Vaslui appears first in a polish document referring to a Lithuanian prince: Jurg Koriatowicz, in 1375. The name Vaslui was also mentioned in 1435, in connection with the accession of Prince Iliaş to the Moldavian throne. The town was burned to the ground in 1439 and 1440, when Tatars invaded Moldavia.
The peak of its importance was in the 15th century, when it was a second-rank capital of Moldavia, during the reign of Stephen the Great, with a population closer to that of the neighbour Iaşi. In 1475, Prince Stephen won his greatest battle against the Ottoman Empire in the Vaslui area. Once the Moldavian capital was moved from Suceava to Iaşi and the southern town of Bârlad became an administrative center of the southern Moldavia, Vaslui declined for about three centuries to a local borough (târg).
There once was a fairly large Jewish community in the city of Vaslui. Their arrival from Galicia during the second half of the 19th century gave a new impetus to the economical development. In 1899, Jews formed 37% of the population, and Vaslui was home to the Vasloi Hasidic dynasty. However, waves of pogroms, associated with the Holocaust as well as emigration to Israel during Romania's communist period largely diminished its presence.
During World War II, the Stephen the Great Monument was relocated from Chişinău to Vaslui.
However, the population grew again steadily after 1968, when the town was proclaimed as the administrative center of Vaslui County, with immigration from the neighbouring countryside, ethnic Romanians and Roma attracted by the industry set up by the Communist regime.
The population decreased again after the downfall of the Communism in 1989, because of the emigration resulting from the decline in life quality.
Today, the majority of the population is of Romanian ethnicity. The Romani minority lives compactly in the southwestern suburbs of Rediu and Brodoc, in the southwestern part of the main town (in the neighbourhoods around Traian Street) and also scattered in the rest of locality. In the 1960s and 1970s nomadic Roma belonging to the Kalderash caste were forcibly settled by the Communists in the northern part of the town, scattered among Romanians. The third ethnic group is that of the Lipovans, who have in the center of the town a church of their Old Believers Christian branch.