Assis-sur-Serre is located some 30 km south-east of Saint-Quentin and 15 km north-west of Laon. It can be accessed by the D35 road from Remies in the west through the heart of the commune and the village and continuing to Pouilly-sur-Serre in the east. There is also the D351 road going north from the village and joining the D642 which continues north to Montigny-sur-Crecy. There is a network of country roads in the commune which is entirely farmland apart from the village.
The Serre river flows through the north of the commune forming part of the northern border but there are no other waterways in the commune other than a small loop off the Serre.
In 2010 the commune had 275 inhabitants. The evolution of the number of inhabitants is known through the population censuses conducted in the town since 1793. From the 21st century, a census of municipalities with fewer than 10,000 inhabitants is held every five years, unlike larger towns that have a sample survey every year.[Note 1]
^At the beginning of the 21st century, the methods of identification have been modified by law No. 2002-276 of 27 February 2002 , the so-called "law of local democracy" and in particular Title V "census operations" which allow, after a transitional period running from 2004 to 2008, the annual publication of the legal population of the different French administrative districts. For municipalities with a population greater than 10,000 inhabitants, a sample survey is conducted annually, the entire territory of these municipalities is taken into account at the end of the period of five years. The first "legal population" after 1999 under this new law came into force in 1 January 2009 and was based on the census of 2006.