KALILAYAN was its first name-derived from the Tagalog term. "Lilay" a kind of palm tree grew once in abundance in the place.

The first Malayan settlers founded the town of KALILAYAN as early as 1591. A Malayan lady of nobility, called b the name Ladya, carried the title - "Queen of Kalilayan". Chinese Salt Commissioner, ChAO-JO-KUA, wrote on 1280 about a certain land called "Lin Sin", which historian presumed to be ancient "KALILAYAN" based on the accounts furnished him by the Chinese traders.

9,000 people, who originated from the offsprings of the Malays, populated Kalilayan. It was made the first capital of the province that called Tayabas, now Quezon.

In the year 1591, the first Christian missionaries, Friars Juan de Placencia and Diego Oreposa arrived in Kalilayan. They built the town along Occidental lines on the west side of the Kalilayan River just opposite the location of the present town of Unisan. The inhabitants enjoyed prosperity until 1609 when a fleet of Moro Pirates suddenly appeared, killed and captured many of its inhabitants and burned the whole town. About 1,000 persons escaped and transferred their settlement to an interior place called Palsabangon now a barrio of the Municipality of Pagbilao.

Kalilayan, being deserted, lost its importance and the capital of the province was transferred to the town of Tayabas, shortly after the raid in 1609. The town of Kalilayan was totally abandoned and became the refuge of lawless people for nearly a century.

The people who formerly evacuated to other places returned and settled to the eastern side of the Kalilayan River, now the very site of the poblacion. The people built a stone castle with watchtower to detect the coming of Moro Pirates. But the Moros did not appear anymore. The people improved the town and converted the old Castle into "Tribunal" or "Presidencia", now the existing Municipal Building.

In 1775, Kalilayan was made a territory of Pitogo when a "Visita" or "Delegacion" was made. In 1876, under the leadership of Teniente Mayor Matias Isaac, the inhabitants petitioned, thru the Spanish Provincial Governor and the Governor General, requesting their separation from Pitogo. The petition was granted and by virtue of the order of the Governor General the assembly was held in Kalilayan on February 18, 1876. In the said assembly, the new town was renamed UNISAN after the boundary limits were esablished and the written constitution was unanimously approved by the representatives.

The name UNISA was derived from a spanish verb "Unir" meaning to unite. In that time, inhabitants were united from strangers from different parts of the island. They wanted to call the town UNISAN, a corrupted Tagalog word in place of Union or Unidos, for sake of euphony.

Another version of the town's name claimed Unisan was derived from the latin words "Uni-Santi" meaning in honor of a Saint. Records discovered from diary of Friar Pedro Bautista, a missionay to Kalilayan and who was later canonized into saint following his martyrdom.