Originally the town had names such as Zanow, Scanowe, Sanowe, Czanow, Tzanow, Sanow. The name itself probably comes from some Slavic term for „hay” (Polish „siano”). The oldest seal of the town comes from the mid 14th century and presents a fish griffin facing right, against a checked background, beneath which there is wavy water and the inscription: „S' SIVITATIS DE SANOWE”.

A fishgriffin was a coat of arms of the Święc family, the gentry ruling in Darłów, Sławno and Polanów and the wavy water symbolizes the river Polnica. Sianów was founded onthe important medieval trade route between Szczecin to Gdańsk. The history of the first settlement in this area is not known; the oldest mention of Sianów comes from the document dated back to 1311 which mentions „villa Sanow” in the description of the borders of Gorzębądź. The next mention comes in the document of the 6 June 1330 drawn up by Sławno Castellan (alderman) Jan Święc, concerning the border agreement between the Cisters Cloister in Buków and the landed estates of Jarosław and Jan von Natzmer. Meanwhile, the first mention of a rural settlement in Sianów comes from a document drawn up in 1335.

Eight years later, on 11 February 1343 Piotr Święca from Polanów, a nephew of the Sławno Castellan, Jan, granted municipal rights under Lubusz law to the settlement.The town was endowed with a village along with meadows, called Nigendorp, later Neues Dorf (probably the area of today‘s ulica Łubuszan) it also obtained the right to use surrounding rivers, lakes and forests. This granting was repeatedly confirmed by the Kamień dukes and the bishops who came into possession of Sianów after the second half of the 14th century.

It is known that in 1353 bishop Johannes from Kamień (Cammin) bought the Polanów land, including the castle and the town of Sianów. The castle and the town constituted so-called pawned land. In 1372 Sianów was the property of the Duke Bogusław VI and Barnima V but from 1386 the ownership was passed to the Kamień diocese. In about 1400, the town was owned again by the dukes and came under the authority of dukes in Darłowo. During the period of Thirty Years’ War the town was seriously impoverished and in the following years its independence was strongly limited, especially after the death of Bogusłam XIV and incorporating (the annexation) of Pomerania by the Province of Brandenburg.

As a result of fights between the gentry and townsmen over  privileges, Sianów fell into a conflict with the national government of Pomerania in 1684 and was degraded to a town of secondary importance. The resolution of 5 June 1662 issued in Kołobrzeg confirmed the municipal rights of Sianów as being part of the Darłowo Castellan’s land but it was denied the judicial and voting rights. At that time the town struggled with financial difficulties. The income was mostly derived from sheep breeding and the animal fairs which took place three times a year. In about 1750, Sianów was one of the most important suppliers of sheep wool stored in Sławno. The crafts were developed to a much lesser extent, though. The Seven Years’ War, plagues and the marching of Napoleon’s army contributed to the fall of the town’s economy. However, in spite of serious damage and financial predicament in 1743 the town bought the lake situated between Skibno and Sianów and in 1780  built an urban mill, a lumber mill, an oil mill and a groats factory. A difficult economic situation was deepened by natural disasters, including the hurricanes which struck the town in 1800 and 1813.

In later years, economic boom brought about economic growth and led to reforms in agriculture. The town developed in terms of industry. On 1 October 1845 a matches making factory owned by Kolbe was opened. In the following years it was considerably developed and in 1861 it already employed 80 people.

In 1871 the Pohl brothers started a second matches manufacture and in 1899 a soap factory was opened. The road network linking Sianów with other neighbouring towns was also developed. The end of World War I was followed by the influx of people from the lands given back to Poland as a result of which 91 new flats were built in Sianów. During World War I Sianów was economically impoverished. Many men were conscripted to the army, there were not enough raw materials and disruptions occured in communication and transport. The lack of food was becoming more acute. 

After the end of WW I, the economic problems continued to plague the town. The large global crisis of 1929-1933 had also  negatively impacted on the town’s economy. The lumber mill, the concrete factory, the tile factory and several other craft manufacturers suspended their production. At that time the Nazi movement intensified its activity. At first the workers of the local factories were opposing the Nazi’s actions. 

The anti-fascist rally organized on 4 July 1932 outside the town hallserves as  proof. After the rally, the Nazi Storm-troopers attacked the workers but were thoroughly beaten by them. After Hitler’s ascent to power, this incident wasthoroughlyinvestigated and the protestors imprisoned for a long time. In 1938 three major factories were operating in Sianów: the matches factory, the factory of agricultural tools and the concrete factory. Therefore, the period before the outbreak of World War II was not particularly favourable to the town.

During the war many forced labour workersstayed in the town and its vicinity: these were Poles, Russians, Dutch and others. Sianów was liberated on 3 March 1945. From the north-east, four Soviet tanks of III Guards Armoured Corps entered the town. They were commanded by Colonel-General AleksiejPanfiłow fighting in the II Bielarussian Front under the command of Marshall Konstanty Rokossowski. Part of the German army surrendered and the remaining soldiers fled in panic towards Koszalin. Right after the liberation the Soviet military commandant’s office was established in Sianów and the Poles staying there elected the first Polish mayor in the mid March 1945. A. Turzyński was appointed for this position. As a result of the war, 15% of Sianow’s houses were destroyed.

In April and May 1945 the first settlers arrived in Sianów. They came mainly from Pomerania and Kujawydistricts and in August the first repatriates arrived intown. In August 1946 the civil administration took back the matches factory occupied by the Polish Army unit and began its reconstruction. The process of rebuilding the factory went smoothly and its opening ceremony coincided with the eighth anniversary of the war’s outbreak.